Downlink routing mechanism

ABSTRACT

The present technology relates to protocols relative to utility meters associated with an open operational framework. More particularly, the present subject matter relates to protocol subject matter for advanced metering infrastructure, adaptable to various international standards, while economically supporting a 2-way mesh network solution in a wireless environment, such as for operating in a residential electricity meter field. The present subject matter supports meters within an ANSI standard C12.22/C12.19 system while economically supporting a 2-way mesh network solution in a wireless environment, such as for operating in a residential electricity meter field, all to permit cell-based adaptive insertion of C12.22 meters within an open framework. Cell isolation is provided through quasi-orthogonal sequences in a frequency hopping network. Additional features relate to apparatus and methodology subject matters relating to Downlink routing mechanisms.

PRIORITY CLAIMS

This application claims the benefit of two previously filed U.S.Provisional Patent Applications entitled “METERING RF LAN PROTOCOL ANDCELL/NODE UTILIZATION AND MANAGEMENT,” respectively assigned U.S. Ser.No. 60/845,056, as filed Sep. 15, 2006, and assigned U.S. Ser. No.60/845,994, as filed Sep. 20, 2006, which are hereby incorporated hereinby reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present technology relates to protocols relative to utility metersassociated with an open operational framework. More particularly, thepresent subject matter relates to protocol subject matter for advancedmetering infrastructure, adaptable to various international standards,while economically supporting a 2-way mesh network solution in awireless environment, such as for operating in a residential electricitymeter field.

BACKGROUND OF THE INVENTION

The general object of metrology is to monitor one or more selectedphysical phenomena to permit a record of monitored events. Such basicpurpose of metrology can be applied to a variety of metering devicesused in a number of contexts. One broad area of measurement relates, forexample, to utility meters. Such role may also specifically include, insuch context, the monitoring of the consumption or production of avariety of forms of energy or other commodities, for example, includingbut not limited to, electricity, water, gas, or oil.

More particularly concerning electricity meters, mechanical forms ofregisters have been historically used for outputting accumulatedelectricity consumption data. Such an approach provided a relativelydependable field device, especially for the basic or relatively lowerlevel task of simply monitoring accumulated kilowatt-hour consumption.

The foregoing basic mechanical form of register was typically limited inits mode of output, so that only a very basic or lower level metrologyfunction was achieved. Subsequently, electronic forms of metrologydevices began to be introduced, to permit relatively higher levels ofmonitoring, involving different forms and modes of data.

In the context of electricity meters specifically, for a variety ofmanagement and billing purposes, it became desirable to obtain usagedata beyond the basic kilowatt-hour consumption readings available withmany electricity meters. For example, additional desired data includedrate of electricity consumption, or date and time of consumption(so-called “time of use” data). Solid state devices provided on printedcircuit boards, for example, utilizing programmable integrated circuitcomponents, have provided effective tools for implementing many of suchhigher level monitoring functions desired in the electricity metercontext.

In addition to the beneficial introduction of electronic forms ofmetrology, a variety of electronic registers have been introduced withcertain advantages. Still further, other forms of data output have beenintroduced and are beneficial for certain applications, including wiredtransmissions, data output via radio frequency transmission, pulseoutput of data, and telephone line connection via such as modems orcellular linkups.

The advent of such variety and alternatives has often required utilitycompanies to make choices about which technologies to utilize. Suchchoices have from time to time been made based on philosophical pointsand preferences and/or based on practical points such as, training andfamiliarity of field personnel with specific designs.

Another aspect of the progression of technology in such area ofmetrology is that various retrofit arrangements have been instituted.For example, some attempts have been made to provide basic meteringdevices with selected more advanced features without having tocompletely change or replace the basic meter in the field. For example,attempts have been made to outfit a basically mechanical metering devicewith electronic output of data, such as for facilitating radio telemetrylinkages.

Another aspect of the electricity meter industry is that utilitycompanies have large-scale requirements, sometimes involving literallyhundreds of thousands of individual meter installations, or data points.Implementing incremental changes in technology, such as retrofitting newfeatures into existing equipment, or attempting to implement changes tobasic components which make various components not interchangeable withother configurations already in the field, can generate considerableindustry problems.

Electricity meters typically include input circuitry for receivingvoltage and current signals at the electrical service. Input circuitryof whatever type or specific design for receiving the electrical servicecurrent signals is referred to herein generally as current acquisitioncircuitry, while input circuitry of whatever type or design forreceiving the electrical service voltage signals is referred to hereingenerally as voltage acquisition circuitry.

Electricity meter input circuitry may be provided with capabilities ofmonitoring one or more phases, depending on whether monitoring is to beprovided in a single or multiphase environment. Moreover, it isdesirable that selectively configurable circuitry may be provided so asto enable the provision of new, alternative or upgraded services orprocessing capabilities within an existing metering device. Suchvariations in desired monitoring environments or capabilities, however,lead to the requirement that a number of different metrologyconfigurations be devised to accommodate the number of phases requiredor desired to be monitored or to provide alternative, additional orupgraded processing capability within a utility meter.

More recently a new ANSI protocol, ANSI C12.22, is being developed thatmay be used to permit open protocol communications among metrologydevices from various manufacturers. C12.22 is the designation of thelatest subclass of the ANSI C12.xx family of Meter Communication andData standards presently under development. Presently defined standardsinclude ANSI C12.18 relating to protocol specifications for Type 2optical ports; ANSI C12.19 relating to Utility industry Meter Data Tabledefinitions; and ANSI C12.21 relating to Plain Old Telephone Service(POTS) transport of C12.19 Data Tables definition. It should beappreciated that while the remainder of the present discussion maydescribe C12.22 as a standard protocol, that, at least at the time offiling the present application, such protocol is still being developedso that the present disclosure is actually intended to describe an openprotocol that may be used as a communications protocol for networkedmetrology and is referred to for discussion purposes as the C12.22standard or C12.22 protocol.

C12.22 is an application layer protocol that provides for the transportof C12.19 data tables over any network medium. Current standards for theC12.22 protocol include: authentication and encryption features;addressing methodology providing unique identifiers for corporate,communication, and end device entities; self describing data models; andmessage routing over heterogeneous networks.

Much as HTTP protocol provides for a common application layer for webbrowsers, C12.22 provides for a common application layer for meteringdevices. Benefits of using such a standard include the provision of: amethodology for both session and session-less communications; commondata encryption and security; a common addressing mechanism for use overboth proprietary and non-proprietary network mediums; interoperabilityamong metering devices within a common communication environment; systemintegration with third-party devices through common interfaces andgateway abstraction; both 2-way and 1-way communications with enddevices; and enhanced security, reliability and speed for transferringmeter data over heterogeneous networks.

To understand why utilities are keenly interested in open protocolcommunications; consider the process and ease of sending e-mails from alaptop computer or a smart phone. Internet providers depend on the useof open protocols to provide e-mail service. E-mails are sent andreceived as long as e-mail addresses are valid, mailboxes are not full,and communication paths are functional. Most e-mail users have theoption of choosing among several Internet providers and severaltechnologies, from dial-up to cellular to broadband, depending mostly onthe cost, speed, and mobility. The e-mail addresses are in a commonformat, and the protocols call for the e-mail to be carried bycommunication carriers without changing the e-mail. The open protocollaid out in the ANSI C.12.22 standard provides the same opportunity formeter communications over networks.

In addition, the desire for increased mesh network operationalcapabilities as well as other considerations including, but not limitedto, a desire to provide improved capabilities for developed so that thepresent disclosure is actually intended to describe an open protocolthat may be used as a communications protocol for networked metrologyand is referred to for discussion purposes as the C12.22 standard orC12.22 protocol.

C12.22 is an application layer protocol that provides for the transportof C12.19 data tables over any network medium. Current standards for theC12.22 protocol include: authentication and encryption features;addressing methodology providing unique identifiers for corporate,communication, and end device entities; self describing data models; andmessage routing over heterogeneous networks.

Much as HTTP protocol provides for a common application layer for webbrowsers, C12.22 provides for a common application layer for meteringdevices. Benefits of using such a standard include the provision of: amethodology for both session and session-less communications; commondata encryption and security; a common addressing mechanism for use overboth proprietary and non-proprietary network mediums; interoperabilityamong metering devices within a common communication environment; systemintegration with third-party devices through common interfaces andgateway abstraction; both 2-way and 1-way communications with enddevices; and enhanced security, reliability and speed for transferringmeter data over heterogeneous networks.

To understand why utilities are keenly interested in open protocolcommunications; consider the process and ease of sending e-mails from alaptop computer or a smart phone. Internet providers depend on the useof open protocols to provide e-mail service. E-mails are sent andreceived as long as e-mail addresses are valid, mailboxes are not full,and communication paths are functional. Most e-mail users have theoption of choosing among several Internet providers and severaltechnologies, from dial-up to cellular to broadband, depending mostly onthe cost, speed, and mobility. The e-mail addresses are in a commonformat, and the protocols call for the e-mail to be carried bycommunication carriers without changing the e-mail. The open protocollaid out in the ANSI C.12.22 standard provides the same opportunity formeter communications over networks.

In addition, the desire for increased mesh network operationalcapabilities as well as other considerations including, but not limitedto, a desire to provide improved capabilities for individual metrologycomponents in an open operational framework, leads to requirements forinterfacing such components with mesh network system applications.

As such, it is desired to provide an improved protocol for advancedmetering infrastructure applications in an open operational framework.

While various aspects and alternative embodiments may be known in thefield of utility metering, no one design has emerged that generallyencompasses the above-referenced characteristics and other desirablefeatures associated with utility metering technology as hereinpresented.

SUMMARY OF THE INVENTION

In view of the recognized features encountered in the prior art andaddressed by the present subject matter, improved apparatus andcorresponding methodology allowing advanced metering infrastructure inan open operational framework have been provided.

In an exemplary arrangement, methodology and corresponding apparatushave been provided to permit transmission of information between autility meter and an operational application through a frequency hoppingnetwork operated in accordance with present protocol subject matter.

In one of its simpler forms, the present technology provides an improvednetwork and meter protocols.

One positive aspect of the present subject matter is that it supportsmeters within an ANSI standard C12.22/C12.19 system while economicallysupporting a 2-way mesh network solution in a wireless environment, suchas for operating in a residential electricity meter field, all to permitcell-based adaptive insertion of C12.22 meters within an open framework.

Another positive aspect of the present subject matter is that itprovides for cell isolation through quasi-orthogonal sequences in afrequency hopping network. Some additional positive aspects relating toa network operated per the present protocol subject matter relate toreal time clock distribution and recovery, uplink routing withoutrequiring a routing table, and the handling of Beacon Requests andRegistered State bit resolving to avoid circular routes.

Still additional positive aspects of the present protocol subject matteras relates to cell or node utilization or management in a mesh network,relate to cell size management, to Number-of-sons'management, to crystaldrift compensation in a mesh network, to broadcast acknowledgementfeatures, and to Traffic Load Control in a Mesh Network.

Additional aspects of the present subject matter relate to Embedded RFenvironmental evaluation tool features to gauge the performance need ofRF transceivers, Downlink routing mechanisms, Outage notification systemfeatures, the use of minimal propagation delay path to optimize a meshnetwork, and operation at the node level of a Discovery Phase in afrequency hopping network.

The present subject matter equally relates to methodology as well asrelated apparatus subject matter, both at the network level and devicelevel. One exemplary present embodiment relates to methodology for anadvanced metering system mesh network with multiple mode downlinkrouting. Such embodiment may preferably comprise establishing a networkincluding a central facility and a plurality of node devices, at leastsome of which node devices comprise metrology devices, and at least someof which node devices comprise cell masters which provide communicationsbetween the central facility and others of the node devices associatedtherewith in respective cells formed by each respective cell master;configuring the network for bidirectional communications between thecentral facility and each of the plurality of node devices; from eachnode device associated with a given specific cell master in a respectivecell formed thereby, providing information to such given specific cellmaster about communications links of each of such node devices in suchrespective cell; selectively sending a monocast downlink message from agiven respective cell master to a specific node device associatedtherewith, using a monocast downlink path determined by such givenrespective cell master based on its received information from all nodedevices associated therewith; and selectively sending a broadcastdownlink message from a given respective cell master to all node devicesassociated therewith.

Another present exemplary methodology embodiment relates to an advancedmetering system frequency hopping mesh network with dynamic multiplemode downlink routing to all node devices thereof. Such embodiment maypreferably comprise methodology for establishing a network including acentral facility and a plurality of node devices, at least some of whichnode devices comprise metrology devices, at least some of which nodedevices comprise cell masters which provide communications between thecentral facility and others of the node devices associated therewith inrespective cells formed by each respective cell master, and at leastsome of which node devices comprise father node devices which provide acommunications link between a respective cell master and others of thenode devices defining son node devices of a given respective father nodedevice in the respective cell of the respective cell master; configuringthe network for bi-directional communications between the centralfacility and each of the plurality of node devices via associations withrespective of the cell masters; at each node device, maintaining aneighbor table containing information about each such node device andits respective neighbor relationships and communications links within arespective cell formed by a respective cell master; from each nodedevice associated with a given specific cell master in a respective cellformed thereby, forwarding the neighbor table thereof to such givenspecific cell master; at each node device associated with a givenspecific cell master in a respective cell formed thereby, updating itsrespective neighbor table when any changes occur to any of itsrespective neighbor relationships or communications links, andforwarding such updated neighbor table thereof to such given specificcell master; selectively sending a broadcast downlink message from agiven respective cell master to all node devices associated therewith inits respective cell; and selectively sending a monocast downlink messagefrom a given respective cell master to a specific node device associatedtherewith in the respective cell formed by such given respective cellmaster, using a monocast downlink path determined by such givenrespective cell master based on received neighbor tables from all nodedevices associated therewith in its respective cell. With suchembodiment, such network dynamically adapts its downlink routing tochanges in communications links among the node devices thereof.

Yet another present exemplary embodiment relates to an advanced advancedmetering system mesh network with multiple mode downlink routing. Suchan embodiment may preferably comprise, comprising a central facility;and a plurality of node devices, at least some of which node devicescomprise metrology devices, and at least some of which node devicescomprise cell masters which provide communications between such centralfacility and others of the node devices associated therewith inrespective cells formed by each respective cell master, with each nodedevice configured for bi-directional communications with such centralfacility. In such embodiment, prefereably each of the node devices isconfigured to provide information to its respective cell master aboutcommunications links thereof in such respective cell; and eachrespective cell master is configured to selectively send a monocastdownlink message from it to a specific node device associated therewith,using a monocast downlink path determined by it based on its receivedinformation from all node devices associated therewith, and is furtherconfigured to selectively send a broadcast downlink message from it toall node devices associated therewith.

In another present exemplary embodiment, an advanced metering systemfrequency hopping mesh network has dynamic multiple mode downlinkrouting to all node devices thereof. Such network preferably comprises acentral facility; and a plurality of node devices, at least some ofwhich node devices comprise metrology devices, at least some of whichnode devices comprise cell masters which provide communications betweensuch central facility and others of such node devices associatedtherewith in respective cells formed by each respective cell master, andat least some of which node devices comprise father node devices whichprovide a communications link between a respective cell master andothers of such node devices defining son node devices of a givenrespective father node device in the respective cell of the respectivecell master, with each of such node devices configured forbi-directional communications between such central facility and each ofsuch plurality of node devices via associations with respective of suchcell masters. In such exemplary embodiment, preferably each of such nodedevices is configured for maintaining a neighbor table containinginformation about its respective neighbor relationships andcommunications links within a respective cell formed by its respectivecell master, for forwarding its neighbor table to its respective cellmaster, for updating its respective neighbor table when any changesoccur to any of its respective neighbor relationships or communicationslinks, and for forwarding such updated neighbor table to its respectivecell master; and each respective cell master is configured forselectively sending a broadcast downlink message to all node devices inits respective cell, and for selectively sending a monocast downlinkmessage to a specific node device associated in its respective cell,using a monocast downlink path determined by it based on receivedneighbor tables from all node devices in its respective cell. With sucharrangement, the network dynamically adapts its downlink routing tochanges in communications links among such node devices thereof.

Still another present exemplary embodiment more particularly relates toa metrology device for use with an advanced metering system mesh networkwith multiple mode downlink routing, and including a central facility, aplurality of node devices, with at least some of such node devicescomprising such metrology devices, and at least some of such nodedevices comprising cell masters providing communications between thecentral facility and others of the node devices associated therewith inrespective cells formed by the respective cell masters, with each nodedevice configured for bidirectional communications with such centralfacility via associations with respective cell masters. Such anexemplary metrology device preferably may comprise a metrology portionconfigured to measure consumption of a utility commodity; a transmitterportion configured to transmit consumption information and other data;and a receiver portion configured to receive information from othernetwork devices in an associated network. Such metrology device portionsare preferably configured for maintaining a neighbor table containinginformation about its respective neighbor relationships andcommunications links within a respective cell formed by its respectivecell master in an associated network, for forwarding its neighbor tableto its respective cell master, for updating its respective neighbortable when any changes occur to any of its respective neighborrelationships or communications links, and for forwarding such updatedneighbor table to its respective cell master, so that such cell mastermay selectively forward downlink messages to the respective node devicesof its cell for arriving at the intended node device.

Additional present features may alternatively and/or further bepracticed with the foregoing exemplary embodiments, whereby additionalpresent embodiments are provided.

Additional objects and advantages of the present subject matter are setforth in, or will be apparent to, those of ordinary skill in the artfrom the detailed description herein. Also, it should be furtherappreciated that modifications and variations to the specificallyillustrated, referred and discussed features, elements, and steps hereofmay be practiced in various embodiments and uses of the present subjectmatter without departing from the spirit and scope of the subjectmatter. Variations may include, but are not limited to, substitution ofequivalent means, features, or steps for those illustrated, referenced,or discussed, and the functional, operational, or positional reversal ofvarious parts, features, steps, or the like.

Still further, it is to be understood that different embodiments, aswell as different presently preferred embodiments, of the presentsubject matter may include various combinations or configurations ofpresently disclosed features, steps, or elements, or their equivalentsincluding combinations of features, parts, or steps or configurationsthereof not expressly shown in the figures or stated in the detaileddescription of such figures. Additional embodiments of the presentsubject matter, not necessarily expressed in the summarized section, mayinclude and incorporate various combinations of aspects of features,components, or steps referenced in the summarized objects above, and/orother features, components, or steps as otherwise discussed in thisapplication. Those of ordinary skill in the art will better appreciatethe features and aspects of such embodiments, and others, upon review ofthe remainder of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present subject matter, includingthe best mode thereof, directed to one of ordinary skill in the art, isset forth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 depicts OSI MODEL subject matter;

FIG. 2A depicts NETWORK ARCHITECTURE subject matter;

FIG. 2B depicts LAYER MODEL subject matter;

FIG. 3A is a block diagram overview illustration of an Advanced MeteringSystem (AMS) and a representation of corresponding methodology thereof,in accordance with the present subject matter;

FIG. 3B illustrates a block diagram of an exemplary meter incorporatinginterface features in accordance with the present subject matter;

FIG. 3C illustrates an exemplary Advanced Metering System deploymentincorporating various of both apparatus and methodology aspects of thepresent subject matter;

FIG. 3D schematically illustrates an exemplary methodology andcorresponding apparatus for transmitting firmware to end devices inaccordance with the present subject matter;

FIG. 3E schematically illustrates an exemplary methodology andcorresponding apparatus for transmitting differing firmware images toselected end devices;

FIG. 3F illustrates a block diagram of the components of a CollectionEngine in accordance with an exemplary embodiment of the present subjectmatter.

FIG. 4 depicts ENTIRE PRESENT SUBJECT MATTER FRAME FOR AN UPLINK MESSAGEsubject matter;

FIG. 5 depicts DATA BLOCKS FOR FEC ENCODING/DECODING subject matter;

FIG. 6 depicts INTERLEAVE TABLE FOR FEC ENCODING/DECODING subjectmatter;

FIG. 7 depicts MPDU AFTER INTERLEAVING AND REED-SOLOMON ENCODING subjectmatter;

FIG. 8 depicts REED-SOLOMON ENCODER subject matter;

FIG. 9 depicts PHY FRAME subject matter;

FIG. 10 depicts PHYSICAL LAYER SERVICES subject matter;

FIG. 11 depicts TIME AND FREQUENCY DIVISION subject matter;

FIG. 12 depicts HYPERFRAME AND CHANNEL SEQUENCE (15 CHANNELS, 10 BASICSEQUENCES) subject matter;

FIG. 13 depicts HYPERFRAME STRUCTURE subject matter;

FIG. 14 depicts PRIMITIVE ELEMENTS FOR PHY-FHSS-NA-915 BASIC HOPPINGSEQUENCES subject matter;

FIG. 15 depicts TS MARGINS AND SUB TIME SLOTS subject matter;

FIG. 16 depicts PROTOCOL TIME KEEPING STRUCTURE subject matter;

FIG. 17 depicts STANDARD ITP TIMESTAMP FORMAT subject matter;

FIG. 18 depicts SYNCHRONIZATION AND LEVELS subject matter;

FIG. 19 depicts HIERARCHICAL SYNCHRONIZATION subject matter;

FIG. 20 depicts RE-SYNCHRONIZATION BETWEEN ENDPOINTS subject matter;

FIG. 21A depicts Potential SYNC fathers and healthy fathers for asynchronized node subject matter;

FIG. 21B depicts Connectivity Degree subject matter;

FIG. 22 depicts DISCOVERING PHASE EXAMPLE WITH BASIC FREQUENCY HOPPINGSEQUENCE NUMBER 0° F. PHY-FHSS-NA-915 subject matter;

FIG. 23 depicts a Table relating to examples of preferred neighborsubject matter;

FIG. 24 depicts CONFIGURATION EXAMPLE subject matter;

FIG. 25 depicts SYNCHRONIZATION PROCESS EXAMPLE subject matter;

FIGS. 26A and 26B respectively depict one example of an InitialConfiguration and one example of a new endpoint, both illustrative ofcircumstances of one endpoint finding a better endpoint forcommunication purposes, per present subject matter;

FIG. 27 depicts RESYNCHRONIZATIONS AND CRYSTAL DRIFT CORRECTIONS IN TIMEsubject matter;

FIG. 28 depicts LOCAL CLOCK DRIFT COMPENSATION ALGORITHM subject matter;

FIG. 29 depicts LOW-PASS FILTER FOR CRYSTAL DRIFT CORRECTION subjectmatter;

FIG. 30A depicts Neighbor Table Management subject matter;

FIG. 30B depicts TYPICAL CRC32 IMPLEMENTATION subject matter;

FIG. 31 depicts TRAFFIC LOAD MONITORING: NODE B IS LISTENING TO THE(N)ACK MESSAGES FROM ITS FATHERS A AND C subject matter;

FIG. 32 depicts present protocol subject matter MESSAGES PRIORITY LISTsubject matter;

FIG. 33 depicts RSSI PDF subject matter;

FIG. 34 depicts RSSI PDF REPORT subject matter;

FIG. 35 depicts MAC FRAME subject matter;

FIG. 36 depicts MAC FRAME TYPE subject matter;

FIG. 37 depicts BEACON subject matter;

FIG. 38 depicts SYNC REQUEST subject matter;

FIG. 39 depicts ACK-NACK-SYNC NACK subject matter;

FIG. 40 depicts SYNC ACK subject matter;

FIG. 41 depicts REQUEST BEACON subject matter;

FIG. 42 depicts MONOCAST DATA subject matter;

FIG. 43 depicts MULTICAST DATA subject matter;

FIG. 44 depicts ITP subject matter;

FIG. 45 depicts DISCOVERY BEACON subject matter;

FIG. 46 depicts OUTAGE subject matter;

FIG. 47 depicts MAC LAYER SERVICES subject matter;

FIG. 48 depicts OVERALL STATES DIAGRAM subject matter;

FIG. 49 depicts LLC PARAMETER DEFAULT VALUES subject matter;

FIG. 50 depicts DELAYED TRUNCATED BINARY EXPONENTIAL BACKOFF FORTRANSMISSION RETRIES IF PACKETS ARE NOT ACKNOWLEDGED subject matter;

FIG. 51 depicts TRUNCATED BINARY EXPONENTIAL BACKOFF FOR TRANSMISSIONRETRIES IF PACKETS ARE NEGATIVELY ACKNOWLEDGED subject matter;

FIG. 52 depicts LLC DUPLICATION TABLE subject matter;

FIG. 53 depicts LLC FRAME subject matter;

FIG. 54 depicts LLC LAYER SERVICES subject matter;

FIG. 55 depicts NET LAYER PARAMETER DEFAULT VALUES subject matter;

FIG. 56 depicts NET DUPLICATION TABLE subject matter;

FIG. 57A depicts Topology subject matter for a Downlink routing example;

FIG. 57B depicts CR Routing Table subject matter for a Downlink routingexample;

FIG. 57C depicts Cell Size Indicator (% of NET_Max_Nb_of EPs) subjectmatter;

FIG. 57D depicts Cell Master actions when cell is full or node not inrouting table subject matter;

FIG. 58 depicts NET FRAME subject matter;

FIG. 59 depicts NET FRAME TYPE subject matter;

FIG. 60 depicts UPLINK FRAME subject matter;

FIG. 61 depicts DOWNLINK FRAME subject matter;

FIG. 62 depicts NEIGHBOR LIST subject matter;

FIG. 63 depicts UPLINK WITH NEIGHBOR LIST FRAME subject matter;

FIG. 64 depicts BROADCAST subject matter;

FIG. 65 depicts CELL OUT NOTIFICATION subject matter;

FIG. 66 depicts BROKEN LINK subject matter;

FIG. 67 depicts OUTAGE subject matter;

FIG. 68 depicts CELL LEAVING NOTIFICATION subject matter;

FIG. 69 depicts NETWORK LAYER SERVICES subject matter;

FIG. 70 depicts MODEL FOR CRYSTAL DRIFT COMPENSATION FEEDBACK LOOPsubject matter;

FIG. 71 depicts a simplified version of the MODEL FOR CRYSTAL DRIFTCOMPENSATION FEEDBACK LOOP subject matter as represented in FIG. 70;

FIG. 72 depicts STEP RESPONSE OF CRYSTAL DRIFT CORRECTION subjectmatter;

FIGS. 73A, 73B, and 73C, respectively, illustrate diagrammatical aspectsof the present subject matter relating to optimization of a meshnetwork;

FIG. 74 depicts TRANSMISSION FAILURE CAUSES AND SOLUTIONS subjectmatter;

FIG. 75 depicts MODEL FOR THE TRAFFIC LOAD AT THE CELL RELAY subjectmatter;

FIG. 76 depicts DATA THROUGHPUT, T(B,A,A) AND PSR (WITH ACKNOWLEDGEMENT)VS TRAFFIC INPUT DENSITY, R(B,A,A) FOR PSRE=0.8 subject matter;

FIG. 77 depicts DATA THROUGHPUT, T(B,A,A) VS PSRE subject matter;

FIG. 78 depicts WAIT TIME AND RANDOMIZATION WINDOW FOR THE(RE-)TRANSMISSION OF A PACKET subject matter;

FIG. 79 depicts COLLISION, PACKET 1 IS LOST subject matter; and

FIG. 80 depicts COLLISION, BOTH PACKETS MIGHT BE LOST subject matter.

Repeat use of reference characters throughout the present specificationand appended drawings is intended to represent same or analogousfeatures, elements, or steps of the present subject matter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various discussion herein makes us of and/or relies on abbreviations andacronyms, having the intended meanings as set forth in the appendedTable of Definitions, which forms part of the present disclosure.

A reference model for interconnection of open systems (referred to asOSI—Open Systems Interconnection) has been described by ISO (theInternational Standards Organization). Such model, represented bypresent FIG. 1, is a functional decomposition of a communication system.In other words, the different layers perform different functions. Alayer N offers services to the layer above N+1, by enhancing theservices offered to this layer N by the underlying layer N−1. Sucharchitecture allows further modifications on one layer without changingthe others. Moreover, it allows compatibility between differentprotocols based on the same principle.

Selected combinations of aspects of the disclosed technology correspondto a plurality of different embodiments of the present subject matter.It should be noted that each of the exemplary embodiments presented anddiscussed herein should not insinuate limitations of the present subjectmatter. Features or steps illustrated or described as part of oneembodiment may be used in combination with aspects of another embodimentto yield yet further embodiments. Additionally, certain features may beinterchanged with similar devices or features not expressly mentionedwhich perform the same or similar function.

The present subject matter network and protocol architecture may bedescribed as based on a tree with four kinds of elements, spread oncells, represented by present FIG. 2A. At the top of such architectureis a Collection Engine, which acts as a central database. It knows allthe cells and their contents, that is, the cell where each meter is andits address. It also collects monthly data from every endpoint and itallows access to any meter in the network. The user can request or senddata to a single meter or broadcast information. The Collection Enginecan communicate with the router of the selected cell by TCP/IP protocolinside Internet network.

In such tree hierarchy, just below the Collection Engine stand routersof the cells, referred to as the Cell Relays. There is one Cell Relayfor each cell and it is the gateway between individual meters in thecell and the Collection Engine. The Cell Relay contains a routing tableof all the meters in its cell. It can also forward data in the twodirections, that is, between the Collection Engine and the endpoints. Italso assumes the role of synchronizing the cell.

At the bottom of such tree are located so-called Endpoints (EPs). Theycan transmit and receive metering information. In addition, each one ofthem can act as a relay for distant endpoints with no additionalhardware.

The last indicated module of such four kinds of elements is the Walk-Byunit, a Zigbee handheld that can communicate with orphans or configurethe subject protocol parameters. This module doesn't itself contain thesubject protocol. It uses the Zigbee part of the register to communicatewith it.

Therefore, the subject network uses 3 media, which are the subject RFlink, a Zigbee RF Link, and a TCP/IP link, all as represented perpresent FIG. 2A.

Present FIG. 2B represents in part the subject protocol, which in partis based on the 3 first layers of the subject layer model, respectivelylabeled as: Physical, Data Link and Network Layer. Such Data Link Layeris further divided into 2 sub-layers, MAC (Medium-Access Control) andLLC (Logical-Link Control). As represented, each layer can communicateby way of the SAP (Service Access Point) with layers just below and justabove. FIG. 2B represents the layer model of an EP (endpoint)incorporating a cell relay module option. On the top of the stack, theAPI layer is communicating with the meter itself to exchange meteringdata or with the Cell Relay application. The left stack represents theCell Master while the right one is the Cell Relay WAN section (or CellRelay board).

The following discussion describes each layer of present FIG. 2B,including their respective functionalities and services.

The Application Layer Interface (API) is not itself part of the subjectprotocol, but from a network point of view it is the layer just above.One main goal of the present subject matter is to transport data fromthe API layer. In one exemplary embodiment, in the AMI network the APIlayer could follow the C12.22 protocol, as discussed throughout thepresent disclosure. However, it is to be understood that the presentsubject matter could also work with another API layer.

The network layer is the highest layer of the present protocol subjectmatter. It is in charge of routing the packets to their finaldestination. To be able to do this it manages a table of its I-hopneighbors obtained through the MAC layer. For an uplink message (towardsthe cell relay (CR)), this table allows the NET layer to send themessage to the best 1-hop neighbor of lower level. The NET layer of thenext endpoint repeats this operation until it reaches the cell relay(lowest level in the cell).

The cell relay (or cell master) NET layer is preferably one exceptionsince it is the only one that can send a message to any endpoint (EP) inthe cell; it is possible because the CR NET has all the neighbor (orfather) tables of the cell and thus is able to send a message with allthe addresses (the whole path) in the header. The CR (or CM) NET layercan also send a broadcast message to all EPs in the cell. For this eachNET layer sends this message to all its sons.

The Logical Link Control (LLC) layer is mainly in charge of repetitionof messages that have not been heard (including in the case ofbroadcast) and of fragmentation of messages that are too long. It alsofilters duplicated messages, in both directions, to not overburden theNET layer or the RF link. Finally, it is often used as just a linkbetween the NET and MAC layers.

The Medium Access Control (MAC) layer handles the largest number oftasks or functions. First, the MAC layer is the synchronization manager.When the power is turned on it tries to find a cell and once connectedto it, it adjusts its level to stay in contact with the best possiblefather.

It is to be understood by those of ordinary skill in the art thatvarious terms may be used to describe certain functional relationships.For example, the terms father or parent may be used interchangeablyrelative to the terms son or child. Choice of such terms herein is notmeant to convey particular limitations or meaning beyond the context inwhich they are presented, as will be understood by those of ordinaryskill in the art.

Among the layers of the subject protocol, the MAC layer is the only oneto have a notion of time. The subject protocol time is divided into TimeSlots (TS) and the MAC layer aligns them with the ones of its fathers,which do the same until operation of the cell relay (which is the timereference in the network). Such synchronization is done through timeinformation included by the MAC layers in the header of all packets. Ifthere is no traffic the MAC layer sends beacons so that its sons canstay synchronized with him.

Another task of the MAC layer is to acknowledge messages received.Positive or negative acknowledgement (ACK or NACK) is possible. Theseare 1-hop acknowledgements. However, if the API layer needs anend-to-end acknowledgement, it needs to insert the request in themessage it sends. The MAC layer also inserts several personal parametersin the header of all messages it sends. When receiving packets from itsneighbors, it extracts these parameters and manages a table of itsneighborhood. Part of this table is communicated to the NET layer.

Finally, the MAC layer computes a CRC (Cyclic Redundancy Check) on thepackets and adds it at the end before giving it to the PHY layer fortransmission or uses it upon reception of a message for error detection.

The Physical (PHY) layer is in charge of transmitting data on the RFlink. It is by default in receiving mode and never decides on its own totransmit. All its instructions, including packets to send, come from theMAC layer. Before transmitting a packet it computes and adds FEC andthen adds a preamble and a header to this protected packet. When itreceives a packet, it removes these 2 additions before delivering thedata to the MAC layer. The physical layer also provides the receivedpacket power (RSSI) and time of reception for the MAC layer. As a lastservice, it can also measure and give the RSSI value on the currentlistening channel.

Present FIGS. 3A through 3C are concerned more particularly withexemplary apparatus and methodology for providing an interface between ameter in an advanced metering system and an application running on sucha system, thereby allowing plug-n-play compatibility (i.e.,interchangeability) of metrology devices in the subject open operationalframework, such as for ANSI standard C12.22 meters, otherwise discussedherein. Also, present FIGS. 3D through 3F relate to exemplary apparatusand methodologies for downloading firmware through a network asdiscussed herein to end devices including utility meters and relays,such as for upgrading firmware in previously installed revenue meters incommunication and cell/node relationships per protocol as otherwisedescribed herein. A viral propagation methodology is disclosed as analternative to at least portions of the broadcast methodology. Varioussuch features may involve downloading firmware in an RF mesh networkthat is arranged in hierarchical layers, or a “tree”-configuredarrangement, as otherwise discussed herein.

FIG. 3A is a block diagram overview illustration of an Advanced MeteringSystem (AMS) in accordance with the present subject matter. AdvancedMetering System (AMS) generally 100 in accordance with the presentsubject matter is designed to be a comprehensive system for providingadvanced metering information and applications to utilities. AMS 100 inpertinent part is designed and built around industry standard protocolsand transports, and therefore is intended to work with standardscompliant components from third parties.

Major components of AMS 100 include exemplary respective meters 142,144, 146, 148, 152, 154, 156, and 158; one or more respectiveradio-based networks including RF neighborhood area network (RF NAN) 162and its accompanying Radio Relay 172, and power line communicationsneighborhood area network (PLC NAN) 164 and its accompanying PLC Relay174; an IP (internet protocol) based Public Backhaul 180; and aCollection Engine 190. Other components within exemplary AMS 100 mayinclude a utility LAN (local area network) 192 and firewall 194 throughwhich communications signals to and from Collection Engine 190 may betransported from and to respective exemplary meters 142, 144, 146, 148,152, 154, 156, and 158 or other devices including, but not limited to,Radio Relay 172 and PLC Relay 174.

AMS 100 is configured to be transparent in a transportation context,such that exemplary respective meters 142, 144, 146, 148, 152, 154, 156,and 158 may be interrogated using Collection Engine 190 regardless ofwhat network infrastructure exists in-between or among such components.Moreover, due to such transparency, the meters may also respond toCollection Engine 190 in the same manner.

As represented by the illustration in FIG. 3A, Collection Engine 190 iscapable of integrating Radio, PLC, and IP connected meters. Tofacilitate such transparency, AMS 100 operates and/or interfaces withANSI standard C12.22 meter communication protocol for networks. C12.22is a network transparent protocol, which allows communications acrossdisparate and asymmetrical network substrates. C12.22 details allaspects of communications, allowing C12.22 compliant meters produced bythird parties to be integrated into a single advanced metering interface(AMI) solution. AMS 100 is configured to provide meter reading as wellas load control/demand response, in home messaging, and outage andrestoration capabilities. All data flowing across the system is sent inthe form of C12.19 tables. The system provides full two-way messaging toevery device; however, many of its functions may be provided throughbroadcast or multicast messaging and session-less communications.

With present reference to FIG. 3B, there is illustrated a block diagramof an exemplary meter 200 incorporating interface features in accordancewith the present subject matter. Meter 200 preferably incorporatesseveral major components including Metrology 210, a Register Board 220,and one or more communications devices. In the presently illustratedexemplary configuration, meter 200 may include such as an RF LANInterface 230 and accompanying antenna 232, and a Zigbee Interface 240and its accompanying antenna 242. In addition, an Option Slot 250 may beprovided to accommodate a third party network or communications module252.

Metrology 210 may correspond to a solid-state device configured toprovide (internal to the meter) C12.18 Blurt communications withRegister Board 220. Communications within meter 200 are conducted viaC12.22 Extended Protocol Specification for Electronic Metering (EPSEM)messages. The meter Register Board 220 is configured to fully supportC12.19 tables and C12.22 extensions. While all meter data will beaccessible via standard C12.19 tables, in order to facilitate very lowbandwidth communications, manufacturers tables or stored procedures areincluded which provide access to specific time-bound slices of data,such as the last calendar day's worth of interval data or othercustomized “groupings” of data.

Meter 200 may be variously configured to provide differingcommunications capabilities. In exemplary configurations, one or more ofGPRS, Ethernet, and RF LAN communications modules may be provided. GPRSwill allow meters to be IP addressable over a public backhaul andprovide more bandwidth than the meter will likely ever require, but mayincur ongoing subscription costs. Ethernet connectivity can be used tobridge to third party technologies, including WiFi, WiMax, in-homegateways, and BPL (Broadband over Power Lines), without integrating anyof these technologies directly into the metering device, but with thetradeoff of requiring external wiring and a two part solution. Ethernetdevices may be used primarily in pilots and other special applications,and they additionally may be ideal for certain high-densityRF-intolerant environments, such as meter closets.

Due to the increased complexity of managing a WAN interface, with itsmore sophisticated link negotiation requirements and TCP/IP(Transmission Control Protocol/Internet Protocol) stack, WAN connectedmeters may include an additional circuit board dedicated to WANconnectivity. Such board if used would preferably interface with meter200 using EPSEM messages and Option Slot 250.

The availability of Option Slot 250 within meter 200 provides theadvantage that it will make meter 200 available for integration withthird party backhauls, such as PLC (Power Line Communications). In orderfor such third party devices to be integrated into AMS 100, on the otherhand, third party devices will need to include both a communicationsboard and a C12.22 compliant relay to couple communications signals fromany proprietary network of the third party to an IP connection.Alternatively, third parties could integrate meter 200 into their ownend-to-end solution.

The communications protocol between meter 200 and respectivecommunications modules 230, 240, and WAN module or optional third partycommunications module 250, follow the C12.22 standards, allowing anythird party to design to the standard and be assured of relativelystraightforward integration.

Communication with the Collection Engine 190 is performed over anInternet Protocol connection. The Wide-Area-Network is a fully routable,addressable, IP network that may involve a variety of differenttechnologies including, but not limited to, GPRS, WiFi, WiMax, Fiber,Private Ethernet, BPL, or any other connection with sufficiently highbandwidth and ability to support full two-way IP communication. Severalassumptions (that is, criteria of the present subject matter) may bemade regarding the IP WAN. Collection Engine 190 is preferablyimplemented so as to be able to communicate directly with otherrespective nodes on the IP WAN. While communications may be conductedthrough a firewall 194, it is riot necessary that such be proxied,unless the proxy is itself a C12.22 node functioning as a relay betweena private IP network and the public IP WAN.

Further in accordance with the present subject matter, the interfacebetween meters and applications manager (IMA Manager) provided by thepresent technology facilitates communications between upper leveldevices including, but not limited to, Collection Engine 190 and thevarious respective meters and other devices within AMS 100. Moreparticularly, the IMA Manager uses a C12.22 manager to create anExtended Protocol Specification for Electronic Meters (EPSEM) messageobject wrapped in an Application Control Service Element (ACSE) object,to send the message to a native network, to receive a response from thenative network, and to return an ACSE object with the EPSEM responseembedded. The IMA Manager preferably would then utilize the IMA for thedevice class in order to build an EPSEM message to be sent to themeters.

The IMA Manager will merge the EPSEM message with any necessary ApTitlesto form an ACSE message and then will pass the ACSE message to theC12.22 Manager. The C12.22 Manager will then send the ACSE message tothe appropriate meters. A response from a meter may be received from thenetwork into the C12.22 Manager, which will parse the ACSE message so asto extract the ApTitle and EPSEM message. Later, the C12.22 Managerreceives a response from the previous ACSE message, parses the ACSEresponse and sends it to the IMA Manager.

The IMA Manager processes an exception response and submits it to anexception manager, which delivers the exception to all systems that havesubscribed to that exception type. The IMA Manager utilizes a Metadatastore to retrieve any information about the calling ApTitle, such as thedevice class and EDL configuration file, and then utilizes the IMA forthe device class to interpret, for example, that an outage has occurred.

The IMA Manager will inform the Exception Manager which respective meterhas experienced an outage. The Exception Manager obtains a list ofsubscribers for the supplied Exception Type from the Metadata Store API,and then sends the message to every notification system that hassubscribed to notifications of the exception's type.

The Advanced Metering System of the present technology provides a series(or plurality) of services (functionalities) to utilities. In its mostbasic implementation, it provides daily feeds of residential interval orTOU (Time of Use) data. Beyond such functionality, it provides poweroutage and restoration notifications, on-demand readings, firmwareupdates, load control/demand response, gas meter readings, and in-homedisplay messages. All of such functions (services) are communicated viathe C12.22 protocol. In order to optimize use of the low-bandwidth RFLAN, selected operations assume use of manufacturer procedures withinthe meter; however, the general C12.22 communication engine of thesystem is not specific to any particular tables, devices, ormanufacturers. In the future, in accordance with the present subjectmatter, as alternate network substrates may become available, the RF LANcan very easily be swapped out with other technologies.

With present reference to FIG. 3C, it will be seen that an exemplaryAdvanced Metering System (AMS) generally 300 deployment has beenillustrated. FIG. 3C illustrates for exemplary purposes only a single RFLAN cell, with twelve respective member nodes organized into threelevels, as well as four directly connected IP meters 370, 372, 374, and376. In such system, all respective meter devices 310, 320, 330, 332,340, 342, 350, 352, 354, 356, 360, 362, 364, 466, 370, 372, 374, and376, Cell Relay 302, and Collection Engine 390, have C12.22 networkaddresses. Collection Engine 390 may in accordance with the presentsubject matter have multiple C12.22 addresses to allow for separateaddressing between different services (functionalities). Meter (orMaster) data management system 391 is not part of the C12.22 network,but preferably it will be implemented so as to communicate over theUtility LAN 392 to Collection Engine 390 via Web Services.Communications between Cell Relay 302 and Utility LAN 392 variouslyinvolve Public Backhaul 380 and firewall 394, in a manner analogous tothat discussed above in conjunction with Public Backhaul 180 andfirewall 194 (FIG. 3A), as well understood by those of ordinary skill inthe art.

The meter data acquisition process begins with the Meter (or Master)Data Management System 391 initiating a request for data. Such operationis done through a web services call to Collection Engine 390 and may beperformed without knowledge of the configured functionality of theend-device. Collection Engine 390 analyzes the request for data, andformulates a series of C12.22 multicast (or broadcast) data requests.Such requests are then sent out either directly to the device (in thecase of an IP connected meter, such as 370), or to Cell Relay 302 thatrelays the message out to all appropriate nodes. Broadcast and multicastmessages are sent by Cell Relay 302 to all members of the cell, eithervia an AMS RF LAN-level broadcast, or by the Cell Relay repeating themessage. For efficiency sake, the use of an RF LAN level broadcast maybe preferred.

Typically these requests are sent as a call to a manufacturer's storedprocedure. In C12.22, stored procedure calls are performed as writes toa predetermined table, e.g. “table 7.” The stored procedure will sendthe default upload configured for such device. For example, a givenmeter may be configured to upload two channels of hourly interval data,plus its event history. Another meter might be programmed to send up itsTOU registers. The stored procedure will require four parameters to befully operative in accordance with the present subject matter: datastart time, data end time, response start time, and response end time.The data start and end time are be used to select which data to send.The response start time and end time are used to determine the windowwithin which the upstream system wants to receive the data. The variousAMS enabled meters of FIG. 3C are preferably field programmable, viaC12.22 tables, as to the type data to be included in a default upload.

When data is sent to Collection Engine 390, is it sent as C12.19 tableself-write with the notification bit set, and the do-not-respond bitset. The result is that per the present subject matter no C12.22acknowledgement is sent in response to the Collection Engine'sbroadcast, nor does the Collection Engine 390 in response to thenotify-write send any response; however, the notify-write effectivelyserves per the present subject matter as an acknowledgement to thereceipt of the broadcast.

The response processing section can use the configured data about an enddevice and the response message from the end device to determine theresults from the device. The response processing section beginsoperation associated with a specific job in a task list, but can beswitched between any active job that is awaiting a response. Suchoperation allows responses that contain logs from the device to beparsed by each job that could be waiting for an action to be completedwithin the end-device. Such also would allow unsolicited messages to beparsed by the IMA code and then later associated with any possible jobs,as determined by the IMA, all in accordance with the present subjectmatter.

While most operations will not require this, the AMS meters will supportchaining a series of EPSEM messages, such as multiple table reads andwrites in a single request. This is functionality that is required inthe C12.22 specification, and will assist in improving the efficiency ofthe system, as it avoids the overhead of sending a separate message foreach EPSEM command. AMS enabled devices will process each requestsequentially, allowing a series of operations to be handled in a singlecommand, each building on the next, such that a read subsequent to awrite would reflect the results of the request write. If a command in anEPSEM chain cannot be completed, remaining commands in the chain arerejected with appropriate error messages, per the present subjectmatter.

When a respective device receives a request, it evaluates the multi-castaddress specified. If the device is a member of the multicast group, itresponds to the request; otherwise, it discards it. Membership indifferent multicast groups is determined via use of C12.22 standardtable 122.

On-demand reading per the present subject matter is similar to the DailyMeter Data Acquisition Process; however, rather than sending a broadcastor multicast request, the on-demand reading process in accordance withthe present subject matter communicates directly to desired respectivemeters. Such process begins with a user initiated an on-demand readthrough an AMS User Interface, or through a web services call from anupstream system. Per the present subject matter, an orchestration layerof the Collection Engine 390 begins by evaluating the current systemload of the communications substrate through which the respective deviceis connected. Requests for an on-demand read from a saturated cell maybe rejected.

Once Collection Engine 390 determines that the request can be honored,it selects per the present subject matter an appropriate communicationserver within the Collection Engine, and submits the command to retrievedata from the device and return it. The communications server forms aC12.22 table read request, encrypts it, and sends it to the devicedirectly, if IP connected, or to Cell Relay 302 for RF LAN connecteddevices. In cases where traffic flows through the RF LAN, the Cell Relaysoftware retrieves the message from the IP backhaul 380, and evaluatesthe message. The destination address (in C12.22 terminology, the calledApTitle) may be stripped off to save bandwidth on the network, relyinginstead on the underlying RF LAN addressing scheme for delivering themessage. The Cell Relay software must also examine whether thedestination ApTitle is still valid within the cell. If the destinationApTitle is no longer valid, the Cell Relay rejects the message,returning an error packet to the Collection Engine. Provided that thedestination is still valid, the Cell Relay software sends the message tothe device across the RF LAN, per the present subject matter.

A protocol stack for the RF LAN advantageously takes the message andconstructs a node path for the message to take before actuallytransmitting the packet. Such pre-constructed node path allows CellRelay 302 per the present subject matter to push a message down throughthe tree of the cell without creating redundant radio messages. IfCollection Engine 390 wants to do an on-demand read to meter 356, itstarts by sending the message to Cell Relay 302. Cell Relay 302 in turnsends out a transmission that will be heard by both respective meters310 and 320 (in the exemplary configuration of present FIG. 3C). Meter320 could go ahead and retransmit the message, but this wouldn't get themessage to meter 356. Instead, it would simply waste bandwidth. With thenode path provided to by the RF LAN protocol stack, meters 310 and 320will hear the message, but per the present subject matter only meter 310will retransmit the message. The retransmitted message of meter 310 willbe heard by both meters 330 and 332, but only meter 332 will be in thenode path, again meaning other parts of the cell (such as meters 350 and352) won't receive a message that would be useless to them. Both meters354 and 356 will hear the message, but it is only addressed to meter356. As such, meter 354, per the present subject matter, will simplyignore it.

Once the message is received at the subject (i.e., intended) meter,whether via RF LAN or via IP, such meter must unpack the request and acton it. The communications module within the device will pull the C12.22message off the network substrate and provide it to the Register Board220 (FIG. 3B). Register Board 220 will decrypt the message based onshared keys, and then respond to the request, encrypting it andreturning it to the calling ApTitle. In the case of the RF LAN, themessage is simply forwarded to the next layer up in the cell. Messagesare forwarded from one layer to the next until they finally reach CellRelay 302, which relays it across the IP backhaul 380 to thecommunications server that initiated the transaction.

FIG. 3D schematically illustrates an exemplary configuration(representing both methodology and apparatus) for implementing one ormore firmware downloads in accordance with the present technology. Whilefor exemplary purposes, most of the discussion herewith refers to suchfirmware downloads as new or upgraded firmware, it is to be understoodthat the present subject matter is equally applicable to, and fullyencompasses, any firmware downloads, regardless of theircharacterization. For example, it might be due to particularcircumstances and/or needs that the firmware to be downloaded for or toa particular device is a return to a previous version of firmware forsuch device. As another example, it might be that the firmware downloadfor a particular device is regarded as being the same version offirmware, or a corrected version thereof, presently resident andoperating with such device, as a technique for in effect rebooting thedevice, or otherwise correcting some corrupted subject matter. All ofsuch variations in the actual constitution and characterization of thenature and/or reasons for the subject downloads, are intended to comewithin in the spirit and scope of the present subject matter.

When new or upgraded firmware is to be installed within a system 400, animage 410 of the firmware to be downloaded will be provided to anAdvanced Metering System (AMS) Collection Engine 412 as a binary imagefile. Further discussion of Collection Engine 412 is included herewithbut for the present it is noted that Collection Engine 412 isresponsible for breaking up the single binary image into a series 420 ofdiscrete blocks 422 that can be distributed across a communicationsarrangement such as an RF LAN, or other media. In an exemplaryembodiment, an ANSI C12.22 compliant media may be used. Such blocks 422will contain a hash or checksum for the block itself to verify theblock's integrity, as well as a block identifier, which is representedin FIG. 3D by the leading and trailing spaces 424 and 426, respectively.

In general, when transferring blocks, each broken down, discrete block422 is in its entirety preferably written into a record in amanufacturer's table for firmware images. End devices 440 are configuredto evaluate such blocks 422 to determine their discrete integrity byusing their block level hashes. The end devices can also validate thatsuch blocks 422 are assembled (that is, reassembled) into the correctorder. Finally, each end device is able to evaluate the integrity of theoverall image by evaluating the CRC (Cyclic Redundancy Check) or hashfor the entire image.

The basic present process for transferring firmware image blocks 422involves in part functionality that is similar to that used for readingdata from meters. A broadcast containing the image blocks 422 is sent tometers 440. Meters 440 indicate, in a manner described further herein,that they have successfully received the image blocks 422. Meters thatdon't respond are retried in a recovery process to make up for anyfailures. Because of the critical nature of firmware images, and thelarge number of blocks involved, some additional control and feedbackmechanisms may be desired in some instances, to logistically handle thevolume of traffic.

Managing the transport of firmware blocks 422 in an environment whichencounters or involves unreliable media becomes critical whentransferring firmware images. In an exemplary configuration, a 384 kbyte firmware image broken into 64 byte blocks will require 6,144 blocksto be transferred completely successfully before it can be loaded. Whentransferring blocks across an RF LAN, for example, it is relativelylikely that at least one node within a given cell will fail tosuccessfully receive a block. Such circumstances are presently addressedin two key manners. First, it is important that blocks be able to betransmitted and received in any order. Second, depending on thepractical reliability of the underlying network, in accordance withpresent subject matter, it may in some instances be practiced tobroadcast a given block several times before resorting to point-to-pointtransfers of image blocks. In an exemplary configuration, it has beenfound that upper level systems, that is the Collection Engine 412 and/ora cell relay 430, should preferably transmit the firmware image at leasttwice, and in some instances three or four times, before resorting topoint-to-point transfer of image blocks.

With further reference to FIG. 3D, a firmware download process beginswith the Collection Engine 412 sending out a broadcast message to alltarget nodes, calling a manufacturer's stored procedure or writing to amanufacturer's table in the device. In such context, a target node maycorrespond to an end device such as meter 448, cell relay 430, or meters440 including representative meters 442, 444, and 446. Such commandindicates to the device the number of firmware blocks it should expectto receive, and that it should now be in firmware download mode.

When in such firmware download mode, the device will report the numberof blocks it has successfully received as part of any daily readrequests. Additionally, being placed in firmware download mode resets tozero a block counter of such device. Moreover, the command includesinstructions to the end devices indicating that no directacknowledgements on the part of the meters should be made. Rather,devices acknowledge such command by reporting their success count aspart of the next interrogation cycle.

Collection Engine 412 is responsible for evaluating, based on thepresence of the firmware block success count, whether all of thetargeted nodes have successfully entered firmware download mode. Nodesthat have not switched to firmware download mode eventually are thenindividually contacted by the Collection Engine 412.

Once the target nodes are in firmware download mode, Collection Engine412 will begin broadcasting firmware blocks 422 to the target nodes 440.As an alternative to transmission of the firmware blocks 422 exclusivelyby Collection Engine 412, it may be desirable to transfer the firmwareimage 410 to the cell relays 430 and then send a command to instructthem to broadcast the firmware image 410 within their respective cell.Such alternative method would be one approach to reducing public carrierback-haul costs and to allowing cell relays to better manage bandwidthwithin their cells.

Completion of the broadcast transfers is a process that may take severaldays, or even weeks, depending on whether it is being done inconjunction with other operations, In any event, after such completion,Collection Engine 412 begins evaluating the block success count of eachof the target nodes. When a node has a complete set of blocks, it willrecord a special event in the meter history log indicating suchsuccessful completion. Most nodes should have a complete set of blocksonce the broadcast transfers are complete. Nodes that are still missingblocks will need to have them transferred point-to-point. Nodes thathave excessive missing blocks after the broadcast process is completemay be flagged for possible maintenance or replacement as beingpotentially defective.

To facilitate point-to-point transfers, Collection Engine 412 will calla second stored procedure in the device. Such second procedure, amanufacturer's stored procedure, will provide a list of missing blocks,by block number. In an exemplary embodiment, the block list will includea predetermined maximum number of blocks, and a status byte indicatingwhether there is more than the predetermined number of blocks missing.For example, the predetermined maximum number of blocks may be set totwenty blocks. In using such method, most meters will receive all blocksand will not need to report on individual blocks; however, those metersthat are missing blocks can be interrogated for a manifest of what theystill require.

Collection Engine 412 will use such missing block data provided by therespective meter 440 to perform point-to-point block transfers. Meternodes that cannot be contacted will be reported to the system operator.Once the point-to-point retries have been completed, the devices can beinstructed to enable the new firmware. The command to activate thefirmware may correspond to a C12.22 manufacturer's stored procedure. Ifa date and time is specified, the device will activate the firmware atthe specified date and time. If no date and time is provided, the devicenormally will be set to activate the firmware download on an immediatebasis.

Successful firmware activation can involve two additional aspects.First, selected metrology devices, i.e., meters, may employ not justone, but a plurality of images related to different aspects of thedevice's operation. In an exemplary configuration, at least threeseparate firmware images may be employed: one for the meter registerboard, another for a neighborhood local area network (LAN)microprocessor, and a third for a home area network (HAN) processor. Ina more specific exemplary configuration, the neighborhood local areanetwork microprocessor may correspond to an RF LAN microprocessor whilethe home area network processor may correspond to a Zigbee processor.Each of such components will have its own firmware image that may needto be updated. Additionally, over the course of time, new metrologydevice versions which require different firmware may be incorporatedinto existing systems. In such case, a given cell may have a mixture ofdevices with different firmware needs. For example, the Zigbee protocolmay be used for communicating with gas meters, in-home displays,load-control relays, and home thermostats.

With reference presently to FIG. 3E, there is illustrated andrepresented an exemplary methodology (and corresponding apparatus) fortransmitting differing firmware images to selected end devices. Asillustrated in FIG. 3E, for the general group of meters 440 illustrated,a first subset of such meters illustrated with a white background (andgenerally represented by meters 460, 462, 464, 466, and 468) support onefirmware image, while a second subset of generally illustrated meters440 illustrated with a grey background (and generally represented bymeters 450, 452, 454, 456, and 458) support another firmware image. As aresult, while meters 462, 464 are under meters 450, 452 in the cellnetwork hierarchy (or tree) and may be able to exchange firmware imageswith each other, the only way meters 462, 464 can receive their firmwareis through meters 450, 452, which in the present example are of anotherdevice type.

In order to handle such exemplary circumstances as represented inpresent FIG. 3E, the firmware image distribution system is independentof the actual device for which the firmware is intended. Put anotherway, when an image is delivered to cell relay 430 and distributed overthe RF LAN, it is distributed to all of the members of the cell thatmatch the broadcast or multicast address used, regardless of whether theimage is compatible with their particular hardware. This means that inaccordance with the present technology, cell members act as hosts forthe firmware. In order to update both types of meters (per the presentrepresentative example), two firmware updates will need to bedistributed. Firmware will be transferred first to meters of the firstsubset (generally represented by meters 460, 462, 464, 466, and 468),and then activated. Secondly, firmware will be transferred to meters ofthe second subset (generally represented by meters 450, 452, 454, 456,and 458), and then activated. Such same mechanism can be used todownload separate firmware images for individual microprocessors withinthe end node, as needed on a case-by-case basis per a specificimplementation of the present subject matter.

Advantageously, in accordance with the present subject matter, thefirmware activation code not only evaluates the integrity of theindividual blocks and the overall firmware image, but it also checkswhether the image is applicable to its actual hardware and for whichhardware it is targeted. In general, the activation command will be sentonly to the appropriate devices by using a multicast group associatedwith the device class. Nevertheless, checking that the image iscompatible with the end device is an appropriate safeguard practiced insome embodiments in accordance with present subject matter.

With reference again to both FIGS. 3D and 3E, it will be observed thatthe various meters or nodes 440 are illustrated as being connected toone another by double-headed arrow lines (representatively illustratedat 470, 472, 474, 476, and 478 in FIG. 3D, and at 480, 482, 484, 486,and 488 in FIG. 3E). Such interconnections schematically illustrate aself generated network formed by the meters 440 themselves per thepresent subject matter, in concert with each other and cell relay 430 asthe individual meters 440 are activated. Because each of the respectivemeters 440 is self contained with respect to the RF LAN formed, anopportunity exists to distribute upgrade software (firmware) among thevarious meters on a viral peer-to-peer basis.

In such foregoing viral peer-to-peer model, a firmware image may bedelivered to exemplary cell relay 430. From there, Collection Engine 412preferably may send a stored procedure command to cell relay 430,indicating that it should distribute such firmware image to the RF LAN.Collection Engine 412 also sends a command to the meter nodes within thecell using a broadcast or multicast message, instructing them that a newfirmware image is available. Once such command is received, cell relay430 makes the firmware available to its local RF LAN processor. Per thepresent subject matter, meter nodes 440 within such cell instruct theirRF LAN processors to begin looking for blocks. At such point, the RF LANprocessors take over the block transfer process. Again, per previouslydiscussed present methodology, such blocks 422 may be sent in any order.

Such presently disclosed viral-type distribution mechanism may be verypowerful and very efficient in that it may be able to make better use ofthe available physical bandwidth. Under such present viral peer-to-peerarrangement, individual meter nodes 440 can grab firmware images orportions of firmware images, from their immediate neighbors or parents,rather than needing to get the data directly from cell relay 430 orCollection Engine 412. As a result, one portion of the cell could beexchanging firmware blocks while another portion of the cell could bepassing various messages between meter nodes 140 and cell relay 130, allwithout impacting each other.

With reference to FIG. 3F, there is illustrated a block diagramrepresentation of exemplary components of Collection Engine 412 inaccordance with an exemplary embodiment of the present subject matter.Collection Engine 412 is a collection of software-based functionalitywhich provides ANSI C12.22 services to the devices that comprise theC12.22 network, including one or more cell relays 430 as well as themetrology and end devices 440. Though such components are preferablysoftware-based, those of ordinary skill in the art will appreciatevarious equivalent forms of implementation, providing the samefunctionality. Conceptually, Collection Engine 412 is comprised of threemajor components, the Orchestration System or Manager generally 520, theMaster Relay/Authentication Host 510, and the communications server orservers (represented by illustrated components 512, 514, and 516).Collection Engine 412 is implemented preferably so as to be able todistribute work across multiple servers 512, 514, and 516 in order tofacilitate scaling.

Within a C12.22 system, the Master Relay 510 is the coordinating processfor the overall system. In order to send or receive C12.22 messages,respective nodes 440 must be registered with the Master Relay. However,before a respective node is allowed to register, it must beauthenticated. The Authentication Host provides such functionality inthe present exemplary embodiment. The Master Relay or station isresponsible for the actual meter data acquisition process, communicatingwith the meter via C12.22 messages.

As will be understood by those of ordinary skill in the art, each of therespective major components of Collection Engine 412 is in turn made upof a series of smaller components and functionality feature sets. TheOrchestration Manager or layer 520 provides coordination between suchcomponents, and presents a unified, single API (Application LayerInterface) to upstream systems. The Orchestration Manager or system 520runs as a single master orchestration service (or functionality) and asa series of agents. Each separate physical server will have anorchestration agent to tie it into the larger system. API requests aredirected to a master orchestration service (or functionality) which inturn works with the orchestration agents to ensure that requested workor methodology is performed or executed.

The Master Relay/Authentication Host 510 will provide standard C12.22registration services/functionality as well as integrated C12.22 networkauthentication functionality/services. One vision for the C12.22protocol is that, similar to DNS (Domain Name Servers), a C12.22 masterrelay may be created which would be shared between multiple utilities,perhaps providing services to an entire region or country. With suchapproach in mind, implementation of a master relay in accordance withpresent technology should provide full support for the use of otherauthentication hosts, and for sending notification messages toregistered hosts. Additionally, the Orchestration Manager or layer 520is preferably implemented so as to be able to receive notifications frommaster relays from other manufacturers, meaning that an implementationof the present subject matter could be realized employing a master relayfrom an outside source.

The representative Communications Servers 512, 514, and 516 providecommunication functionality with devices, such as to parse and translatesuch communications, and post or return data as necessary. CommunicationServers 512, 514, and 516 thus preferably may comprise a series ofservices/functionality to accomplish such overall functionality per thepresent subject matter. Within Communications Servers 512, 514, and 516are a series of major components: a meter communications host, a dataspooler, and an exception event manager. The meter communications hostis responsible for listening for network communications and sendingnetwork communications. It is the component that both “speaks” C12.22and “interprets” C12.19 table data. The data spooler and the exceptionevent manager provide mechanisms for streaming meter data and exceptionevents, respectively, to upstream systems.

FIG. 4 shows per present subject matter an example of an entire frame ofthe subject protocol for an uplink message, that is, data messages sentfrom a synchronized endpoint to a cell relay. Each field is explainedherein per the frame description section for each respective layer.

Per the present subject matter, there are several proposed physicallayers. All use Frequency Hopping Spread Spectrum technique. Each one ofthem is intended to be used for a specific market location (such aseither for North America or for Europe) and inside a specific band, andfollows the regional or local required regulations.

The physical layer termed PHY-FHSS-NA-915 is intended to be used in the902-928 MHz ISM band in North America. It complies with pertinent FCCregulations, namely parts 15.35, 15.205, 15.209, and 15.247, and in apreferred embodiment may encompass 52 channels. Other particulars of thetransmitter and receiver specifications for such 915 MHz Band, for NorthAmerica, as well as the channel allocations thereof, are well known tothose of ordinary skill in the art from the pertinent regulations,without requiring additional discussion herewith.

The physical layer termed PHY-FHSS-NA-2400 is intended to be used in the2.4 GHz ISM band in North America. It complies with pertinent FCCregulations, namely parts 15.35, 15.209, 15.247 and 15.249, and in apreferred embodiment may encompass 16 channels. Other particulars of thetransmitter and receiver specifications for such 2.4 GHz Band, for NorthAmerica, as well as the channel allocations thereof, are also well knownto those of ordinary skill in the art from the pertinent regulations,without requiring additional discussion herewith.

The physical layer termed PHY-FHSS-EU-868 is intended to be used in the868 MHz ISM band in the European Union. It complies with pertinent ETSIregulations, namely sections 300 220 and ERC 70-03. Other particulars ofthe transmitter and receiver specifications for such 868 MHz ISM band inthe European Union are also well known to those of ordinary skill in theart from the pertinent regulations, without requiring additionaldiscussion herewith.

The physical layer termed PHY-FHSS-EU-2400 is intended to be used in the2.4 GHz ISM band in the European Union. It complies with pertinent ETSIregulations, namely sections ETSI 300 228 and ERC 70-03, and in apreferred embodiment may encompass 16 channels. Other particulars of thetransmitter and receiver specifications for such 2.4 GHz ISM band in theEuropean Union are also well known to those of ordinary skill in the artfrom the pertinent regulations, without requiring additional discussionherewith.

With reference to the PHY Layer, this describes the adjustable (that is,“tweakable”) parameters of such PHY layer. For PHY_Synch_Length: Thelength in bits of the Synch field, the synchronization bit sequence ofthe PPDU. For PHY_SFD_Value: The value of the SFD field of the PPDU.This value should be chosen with appropriate auto-correlation propertiesto enable a reliable start of frame detection. The recommended valuesare given in the following table. The most significant bit ofPHY_SFD_Value is sent first on the air interface, just after thepreamble. These SFD values have sufficient orthogonality to be used withthe purpose of isolating several coexisting networks. ForPHY_RSSI_Average_Time: The averaging time to perform the RSSI reading.

Value Number PHY_SFD_Value 1 0xB127 (default) 2 0x9363 3 0x263D 4 0x23E55 0x0BCD 6 0x871B 7 0x19A7 8 0x94F8 9 0x1A67 10  0x25E3 11  0x8DD2 12 0x1CB6 13  0x895B 14  0x26F1 15  0x258F 16  0x2C76

PHY Parameter Default Values may be preferably regarded as follows. ForPHY_Synch_Length: 32 bytes; for PHY_SFD_Value: 0xB127 (with the mostsignificant bit being the first sent on the air interface); and forPHY_RSSI_Average_Time: 1 ms.

As far as medium access parameters, time and frequency divisions aremanaged with the use of the Slotted Aloha technique (details of whichare well known to those of ordinary skill in the art). Time is generallydivided into identical Time Slots (TS) and at each change of TS (TimeSlot), the frequency hops from one channel to another channel accordingto a frequency hopping sequence. MAC layer is in charge ofsynchronization. Since typically traffic at any point in time is low,Slotted Aloha technique is an appropriate selection. Each meter is bydefault in receiver mode and pushes data on the medium when it wants tospeak. Such method generally may be producing a relative number ofcollisions but, in normal operating conditions, internal collisions areexpected to be lower than collisions due to the noisy environment of ISMbands. However, the present protocol intentionally manages repetitionsto compensate for such phenomenon.

The physical layer can give to the layers above it a snapshot of thelink quality between the endpoint and the sender of the message. Thephysical layer does this by measuring the Received Signal StrengthIndicator (RSSI) during the message reception. Such reading is processedduring the reception of the synchronization bits, which are analternating zero-one sequence. This is an average reading overPHY_RSSI_Average_Time ms. When the physical layer detects the StartFrame Delimiter, it stops the reading of RSSI and saves the value togive it later to the MAC layer together with the received message.

The RSSI is also a measurement of the interference level on a givenchannel when used outside the reception of a present protocol networkmessage. This is used for instance in the environment analysisprocesses.

In all cases, the RSSI value sent to the MAC layer by the physical layeris preferably not a mere reading of the RSSI register of a transceiverchipset. The value is preferably processed to reflect the actual powerlevel of the incoming signal, that is, LNA gain and filter insertionlosses should be taken into account in the RSSI value forwarded to theMAC layer.

The RSSI is formatted as a single signed byte. The value is given indBm. Its theoretical range is therefore from −128 dBm to +127 dBm.

Data is preferably transmitted in the little endian format (LeastSignificant Bit first). Bit ordering is also done with the leastsignificant bit first.

The Forward Error Correction (FEC) used by the physical layer is aninterleaved Reed-Solomon (38,28) code with symbols in Galois Field(GF)(256). The steps for encoding are:

-   -   If the MPDU length is greater than 28 bytes, divide the MPDU in        N 28-byte blocks. If the MPDU length is not a multiple of 28,        the last block will be shorter.    -   For each one of these blocks, compute the 10 redundancy bytes        with the RS (38.28) code. If necessary, the last block is zero        padded to allow the use of the same computation algorithm for        all the blocks.    -   Write the data blocks and the redundancy bytes in the interleave        table. Each block occupies a row in this table.    -   Build the LPDU starting from the first column of the interleave        table and finishing with the last one. The zeros introduced in        the last block serve the purpose of encoding only, they are not        sent on the air interface. The length field of the PHY header        includes the FEC but doesn't include the zero padding, it is        computed with        Length=38*N−(number of zeros added in block N)

FIG. 5 illustrates the fragmentation of the MPDU into blocks prior toReed-Solomon encoding; in other words, FIG. 5 shows data blocks for FECencoding/decoding.

FIG. 6 shows a present exemplary interleave table structure for FECencoding/decoding. Such interleaving is done byte-wise, that is, thefirst byte of block 1 is sent, then the first byte of block 2. Suchoperation is continued until the last FEC byte is sent. After suchReed-Solomon encoding and interleaving, the redundancy bytes are at theend of the frame as shown per present FIG. 7.

The steps for decoding are the same as for such encoding but in reverseorder:

-   -   Fill in the interleave table beginning with the first column.        From the length field of the PHY header and the block size, it        is possible to know where zero padding needs to be inserted.    -   For each row in the interleave table, correct the errors with        the RS (38,28) code.    -   Strip off the FEC and reassemble the MPDU.

The Reed-Solomon coding preferably used in the present protocol is an RS(38,28) code. It is a shortened version of an RS (255,245) code withsymbols in GF (256). This means that the symbols of the code are bytesand that for each block of 28 bytes, there are appended 10 additionalredundancy bytes. This code has a Hamming distance of 11 and allows thecorrection of 5 erroneous bytes per block.

More particularly, each byte of the message is considered as an elementof GF (256). Reference herein to such elements will be as symbols. Allthe operations made with such symbols (addition, subtraction,multiplication and division) should be made according to the additiveand multiplicative laws of the Galois field GF (256), constructed withthe primitive polynomial p(x)=x⁸+x⁷+x²+x+1. A symbol of GF (256) hasseveral useful representations: a binary representation{b₇b₆b₅b₄b₃b₂b₁b₀}, a polynomial representationb₇α⁷+b₆α⁶+b₅α⁵+b₄α⁴+b₃α³+b₂α²+b₁α+b₀ and an exponential representationα^(m). In the last two representations, α is a primitive element suchthat p(α)=0. The binary or polynomial representation is useful foraddition and the exponential representation is useful formultiplication. All GF(256) elements, except zero, have an exponentialrepresentation, the complete field can written as the set GF(256)={0, 1,α, α², α³, . . . , α²⁵³, α²⁵⁴}. The conversion between the tworepresentations is done with a look-up table. For the implementation ofthe encoder/decoder, it is necessary to add and multiply the symbols inGF(256). Addition is readily performed with the binary or polynomialrepresentation of the symbols. The operation is equivalent to a modulo 2addition of each bit, for instance0010 1100+1000 1111=1010 0011

Multiplication is relatively more difficult because a conversion to theexponential form of the symbol is necessary. A look-up table ispreferably used for such purpose. As an example, the following amultiplication of the two symbols of the previous example0010 1100×1000 1111

The first symbol (44) has the exponential representation α¹⁹⁰, thesecond symbol (143) has the exponential representation α⁹⁰. The productis α¹⁹⁰α⁹⁰=α²⁸⁰=α²⁵⁵⁺²⁵=α²⁵ because α²⁵⁵=1. Such table can be used inthe other way to convert the result in binary form and obtain44*143=226.

With reference to the present Reed-Solomon Encoder as preferablyimplemented in accordance with present subject matter, conforming to theestablished convention in coding theory, one would use here thepolynomial representation for the messages. That means that the present28-symbol data block, {u₂₇, u₂₆, u₂₅, . . . u₁, u₀}, can be writtenu(x)=u₂₇x²⁷+u₂₆x²⁶+ . . . +u₃x³+u₂x²+u₁x+u₀. The symbol u₂₇ is the firstsymbol written in the interleave table, which is filled from left toright. The 38-symbol code word (message+redundancy symbols) can bewritten c(x)=c₃₇x³⁷+c₃₆x³⁶+ . . . +c₃x³+c₂x²+c₁x+c₀. The Reed-Solomonencoding process is equivalent to a division of the message by agenerator polynomial G(x). This can be implemented with a linearfeedback shift register as shown in present FIG. 8. This is similar to,but different from a conventional CRC encoder. The difference is that inthe present subject matter each multiplication by a coefficient and eachaddition has to be understood as multiplication and addition in GF(256).

In the shift register implementation represented in present FIG. 8, theg_(i) are elements of GF (256). The shift register is first initializedwith zeros. The symbols of the message are then shifted into theregister, beginning with u₂₇ and finishing with u₀. After the whole datablock has been pushed into the shift register, the content of theregister is the remainder of the division. These symbols are the FECsymbols and they are appended to the initial message in the followingway{u₂₇, u₂₆, u₂₅, . . . u₁, u₀, p₉, p₈, p₇, p₆, p₅, p₄, p₃, p₂, p₁, p₀}

As with conventional CRC computing, the multiplicative factors in theshift register are given by the coefficients of a polynomial:G(x)=g ₀ +g ₁ x+g ₂ x ² +g ₃ x ³ +g ₄ x ⁴ +g ₅ x ⁵ +g ₆ x ⁶ +g ₇ x ⁷ +g₈ x ⁸ +g ₉ x ⁹ +x ¹⁰

In our case this polynomial is:G(x)=(x−α)(x−α ²)(x−α ³)(x−α ⁴)(x−α ⁵)(x−α ⁶)(x−α ⁷)(x−α ⁸)(x−α ⁹)(x−α¹⁰)Once such polynomial is developed to find the gi coefficients, theresult isG(x)=α⁵⁵+α¹²⁰ x+α ⁵⁷ x ²+α¹²⁶ x ³+α¹³⁶ x ⁴+α¹⁹⁸ x ⁵+α¹²⁵ x ⁶+α¹⁰⁴ x⁷+α²⁴ x ⁸+α⁷⁶ x ⁹ +x ¹⁰

The first step of the Reed-Solomon decoding process is the syndromecomputation. For a 38-byte received block, 10 syndromes are preferablycomputed. If the received message is {r₃₇, r₃₆, . . . r₂, r₁, r₀}, thefollowing polynomial representation applies:R(x)=r ₃₇ x ³⁷ +r ₃₆ x ³⁶ + . . . +r ₁ x+r ₀

The 10 syndromes {S₁, S₂, . . . , S₁₀} will be computed with R(x) asshown below:S _(i) =R(α^(i)), for 1≦i≦10

If the 10 syndromes are equal to zero, there is no detectable error inthe message and the decoding process stops here. There is of course apossibility for an undetectable error but preferably this will bedetected later by the CRC.

The second step of the Reed-Solomon decoding process is the computationof the error locator polynomial. This polynomial has the following form:ELP(x)=1+ELP₁ x+ELP₂ x ²+ . . . ELP_(v) x ^(v) ,v≦5

The degree of such polynomial is not known beforehand because it dependson the number of errors in the received message. To compute thecoefficients of ELP(x), an iterative algorithm is used, theBerlekamp-Massey algorithm. A pseudo-code description of such algorithmis as follows:

Input: S₁, S₂, . . . , S₁₀ (the syndromes) Initialization: Len=0ELP(x)=1 (the current error locator polynomial) PELP(x)=1 (the previouserror locator polynomial) j=1 d_(m)=1 (the previous discrepancy) for k=1to 10$d = {S_{k} + {\sum\limits_{i = 1}^{Len}{{ELP}_{i}S_{k - i}\mspace{34mu}\left( {{compute}\mspace{14mu}{discrepancy}} \right)}}}$if d=0 (no change in polynomial) j=j+1 else if 2Len ≧ k then ELP(x) =ELP(x) − dd_(m) ⁻¹x^(j)PELP(x) j=j+1 else temp(x)=ELP(x) (temporarystorage) ELP(x) = ELP(x) − dd_(m) ⁻¹x^(j)PELP(x) Len=k−LenPELP(x)=temp(x) d_(m)=d j=1 end end end

Once the error locator polynomial has been computed, the roots of suchpolynomial are found. To find such roots, ELP(x) is computed for everynon zero z in GF (256) until v roots are found, where v is the degree ofthe error locator polynomial. If less than v roots are found, the errorsare uncorrectable and the whole packet must be rejected. The inverse ofthe roots of ELP (x) are called the error locators. An error locatorindicates the position of an error in the message, as follows.

$\begin{matrix}{{{{ELP}(Y)} = 0},{Y \in {{GF}(256)}}} \\{\left. \Rightarrow X \right. = {\frac{1}{\; Y}\mspace{14mu}{is}\mspace{14mu}{the}\mspace{14mu}{error}{\mspace{11mu}\;}{locator}}} \\{{\left. \Rightarrow{{if}\mspace{14mu} X} \right. = \alpha^{\; k}},{{the}\mspace{14mu}{symbol}\mspace{14mu}{in}\mspace{14mu}{position}\mspace{14mu} k\mspace{14mu}{is}\mspace{14mu}{corrupted}}}\end{matrix}$

As an example, it is found that ELP(α²³⁵)=0 α⁻²³⁵ is an error locator.The inverse of α²³⁵ is computed, which is α⁻²³⁵=α²⁵⁵α⁻²³⁵=α²⁰. It isconcluded in such exemplary instance that the symbol in position 20 iscorrupted, that is, r₂₀ in the sequence {r₃₇, r₃₆, . . . r₂, r₁, r₀}. Atthis stage of the decoding process, if the error pattern is correctable,the positions of all corrupted symbols in the block are known. The setof error locators is:{X ₁ , X ₂ , . . . X _(v)}, ELP(X _(i) ⁻¹)=0

One present technique to potentially speed up the computation of theroots of ELP(x) is to try only the elements of GF (256) that will pointto errors within the block, that is:{1, α⁻¹, α⁻², α⁻³, . . . , α⁻³⁷}.

The next step preferably per the present subject matter is to correctthe errors. Such involves computing the error evaluator polynomial,defined by the following:EEP(x)=S(x)ELP(x)mod(x ¹⁰)

where a new polynomial is introduced whose coefficients are thesyndromesS(x)=S ₁ +S ₂ x+S ₃ x ² + . . . +S ₁₀ x ⁹

The mod (x¹⁰) in the definition of EEP(x) means that all terms of degree10 or higher are discarded. It also involves use of a polynomial calledthe formal derivative of the error locator polynomial, as follows:DER_ELP(x)=ELP₁+ELP₃ x ²+ELP₅ x ⁴

The error on the symbol indicated by the error locator X_(k) is given by

${{Error}(k)} = \frac{{EEP}\left( X_{k}^{- 1} \right)}{{DER\_ ELP}\left( X_{k}^{- 1} \right)}$

The corrected symbol isNew value of symbol=Old value of symbol+Error(k)

The format of the physical layer frame is represented by present FIG. 9.The Synch field of the Preamble (see FIG. 9) of the physical layer frameis composed of an alternating zero-one sequence. It is intended to allowthe remote receiver to detect the presence of an incoming message and toperform a data clock recovery. It indicates an imminent frame and it isalso advantageously used by the receiver to measure the power strength(RSSI) of the received signal. The Synch field length is given by thePHY layer parameter PHY_Synch_Length. The default value for thisparameter is 32 bits (4 bytes).

The Start of Frame Delimiter (SFD) field (see FIG. 9) is two-bytes long.It signals the end of the preamble and the beginning of the PHY header.It has a predefined fixed value given by the PHY layer parameterPHY_SFD_Value. If several networks operated by different utilitiescoexist in the same area, it is possible per the present subject matterto assign a specific SFD value to each network. As only the packets withthe right SFD value are considered, this will prevent an endpoint fromprocessing the packets intended for another network. When used inconjunction with the Utility ID, such feature advantageously increasesthe possible number of networks coexisting in the same area.

Still further per present features, as an option the SFD detection canbe configured per present subject matter to accept the packet if only 15of the 16 bits match PHY_SFD_Value. When used with FEC, such alternativeserves to increase noise immunity per the present subject matter.

With reference to the PHY Header (see FIG. 9), the Utility ID bits allowthe distinction of the different utilities that use this protocol at thesame location. It advantageously avoids a utility entering into a cellbelonging to one of its competitors. When the UID of the packet doesn'tmatch with the UID defined in the endpoint, the received packet isdiscarded. The subsequent two bits of the packet are preferablyreserved, that is, not used initially, to preserve future uses, andshould preferably be set to 0.

The Length bits indicate the length of the MPDU, in bytes. When theindicated number of bytes is received, the physical layer (if theoutcome of the FEC decoding is a valid packet) indicates the arrival ofa complete frame to the MAC layer, which will verify its integrity(CRC). Then the PHY layer, which is by default in receiving mode, looksfor the next preamble.

To add some robustness to the PHY header that has no CRC, the fieldsUID, RSD and Length are preferably complemented bit by bit, per thepresent subject matter. If the complemented fields don't match with theassociated fields, the received packet is discarded.

With reference to the Frame Body (see FIG. 9), the MAC Protocol DataUnit (MPDU) contains all the information required at the MAC layerlevel. The Frame Body also contained redundant bytes, added according tothe FEC algorithm explained hereinabove.

In the description herein of each layer in the document, interface andservices with the upper layer are presented in the same way. A figureshows the different interactions between the layers. There are threedifferent flows of information between them, namely, requests,confirmations, and indications. These different internal communicationsare then explained.

Regarding Requests, Layer-Name_Request-Name, are the services offered bylayer to the layer above. When the service is not needed by the layerjust above but by the one, two, or three times upper, the in-betweenlayers should forward the service. The user may optionally shortcut themiddle layers if desired for particular embodiments.

Regarding Confirmations, Layer-Name_Confirmation-Name, are answers ofthe requests. Every request has a unique confirmation per the presentsubject matter. Confirmations are not always sent immediately after arequest; it depends on the service. The preferred approach is establisha number to the request and then give the same number when theconfirmation is sent, in order to avoid misunderstanding. Note that thenumber associated to the request need not be correlated to the ID of theframes to send.

Regarding Indications, Layer-Name_Indication-Name, are sent by a layerto the layer above, they are not a response to a request, they usuallyreport an event. The layer that sends an indication doesn't expect anyconfirmation.

Further with respect to interfacing and services among layers, thephysical layer is in charge of sending and receiving radio packets onthe medium. Therefore, per present subject matter, by default, thephysical layer is in receive mode. As soon a packet is transmitted, thephysical layer switches back to the receive mode. Channel changes arepreferably requested by the upper layer, the MAC layer. The physicallayer also manages the transceiver concerning its communicationinterface, channel calibration, PLL lock status, RSSI reading, and modeselection. In the transmit (Tx) mode, it computes an FCS (based on RScode) and adds it to the packet; it then adds a physical header(PHY-Header), and a preamble. Finally, it codes and modulates the radiopacket at the required rate and frequency. In the Receive (Rx) mode, itlistens to the medium until it finds a preamble. As soon as itrecognizes the Start of Frame Delimiter at the end of the preamble, itsaves the reception time (SACT) and measures the input power strength(RSSI). Then it demodulates and decodes the radio packet. After removingthe preamble and the header, it corrects the packet if needed (and if itis possible) and indicates to the MAC layer the arrival of a newmessage. Dating of the message should be accurate enough to enable theproper operation of the protocol, as otherwise discussed herein withreference to the MAC layer.

The physical layer proposes different services, as represented by FIG.10.

In conjunction with PHY Requests, referenced as PHY_Request_Send, theobjective is to send a packet over the RF link at a specified channeland at given time. The requisite Input arguments are: MPDU, Channel, andTime to send the packet. The operation may be described as follows. TheMAC layer requests from the PHY layer to send a packet. The channel isgiven with the request. The PHY layer sends the packet at the timeindicated by the MAC. Timing information can be given by severalalternative ways per the present subject matter, which the user is freeto decide. As soon as the packet is transmitted, the physical layerswitches back to receive mode on the time slot's default channel andconfirms to the MAC layer the status of the transmission.

In conjunction with PHY Requests to change channel, referenced asPHY_Request_Change_Channel, the objective is to change the listeningchannel. The requisite Input arguments are Channel information. Theoperation may be described as follows. The MAC layer requests from thePHY layer to change the default receiving channel immediately.

In conjunction with PHY Requests to Read an RSSI value, referenced asPHY_Request_Read_RSSI, the objective is to read the RSSI value. Thereare no requisite Input arguments. The operation may be described asfollows. The subject request asks for the PHY layer to read immediatelythe RSSI instantaneous value on the current channel.

In conjunction with PHY Requests to either start or stop, referenced asPHY_Request_Start_Stop_Rx, the objective is either to Start or Stoplistening on the current default channel. The requisite Input argumentsare whether to Start or Stop. Such operation may be described asfollows. This request asks for the PHY layer to start or stopimmediately to listening on the current default channel. Preferably, perpresent subject matter, the Stop request is mainly used when a poweroutage is detected to save energy.

In conjunction with PHY Confirmation, referenced asPHY_Confirmation_Send, the objective is the Answer of aPHY_Request_Send. The requisite Output arguments are the Status byte.The operation may be descried as confirming the status of a transmittedmessage. It can be Send_Ack, PLL_Unlock or any kind of errors.

In conjunction with PHY Change Confirmation, referenced asPHY_Confirmation_Change_Channel; the objective is the Answer of aPHY_Request_Change_Channel. The requisite Output arguments are theStatus byte. The operation may be descried as confirming the status ofthe Change Channel Request. It can be Ack, PLL_Unlock or any kind oferrors.

In conjunction with PHY Read Confirmation, referenced asPHY_Confirmation_Read_RSSI, the objective is the Answer of aPHY_Request_Read_RSSI. The requisite Output arguments are RSSI. Theoperation may be described as the PHY layer returns the current RSSIvalue. The value is one signed character, expressed in dBm.

In conjunction with PHY Confirmation of Start or Stop Reception,referenced as PHY_Confirmation_Start_Stop_Rx, the objective is theAnswer of a PHY_Request_Start_Stop_Rx. The requisite Input arguments arethe Status byte. The operation may be described as just confirms if therequest has been well performed.

In conjunction with certain PHY Indications, referenced asPHY_Indication_Received, the objective is to Indicate the reception ofan incoming packet. The requisite Output arguments are MPDU, SACT, andRSSI. The operation may be described as after the reception of amessage, the PHY layer removes its header and gives the MPDU to the MAClayer. The PHY layer also indicated the RSSI measured during thereception and the SACT value (see MAC layer chapter for the definitionof the SACT) when the SFD has been captured.

The Data Link layer is divided into two sub-layers, the MAC layer andthe LLC layer. The MAC layer has various tasks, by which it organizesdata transmission on the RF channels and manages the synchronization.

Specifically, with reference to Cyclic Redundancy Check (CRC), the firstrole of the MAC layer is to detect errors in the received datagram.Prior to transmission, the MAC layer computes a CRC based on the packetto be transmitted, and it adds them at the end of the packet. Due tothat function, at the reception of the packet, the MAC layer has theability to accept or discard messages depending on the value of thecodes.

The second task of the MAC layer is the assembling and disassembling ofthe datagram. The MAC layer knows what kind of message the meter hasreceived, who has sent (SA) it and for whom it is addressed (DA).Therefore, the MAC layer also performs acknowledgements. When a messageis received, an acknowledgement message is transmitted back in the sametime slot with an ACK or NACK argument, and with the frame number. Thisacknowledgement message will not be further acknowledged; the MAC layerprovides acknowledgment at each hop of the message but there is no MACend-to-end acknowledgment.

Another task is the neighborhood management. Due to the receivedmessages and their header, the MAC layer manages a table, where severalindications about the 1-hop neighbors are saved. When a neighbor hasn'ttransmitted anything for a long time, it is considered as having leftand it is removed from the table. This table is used for severalpurposes, like synchronization. It is also shared with the network layerto allow message routing.

Another task performed by the MAC layer is the management ofsynchronization. The MAC layer readjusts the start of time slots inlistening traffic and in receiving synchronization packets. Because itknows time division and the frequency hopping sequence, it can choosethe channel to use. To synchronize with the other endpoints, if there isno traffic, the MAC layer sends periodic beacons.

The following describes parameters of the MAC layer, more specificallylisting a variety of parameters which are adjustable (that is,tweakable). It provides a description of each parameter and somecomments on how to modify their value (effect, limits . . . ) and/ortheir relations with other parameters. Depending on the location and theband in which the present protocol subject matter operates, the defaultvalues of the MAC parameters change, as will be understood by those ofordinary skill in the art without requiring additional detaileddiscussion thereof.

MAC_Beacon_Period_SYNC

-   Description: The period of the beacons sent by a synchronized    endpoint when it has no other messages to send. It corresponds to    the maximal allowed period of radio inactivity.-   Comments: The value of this parameter depends on the clock drift and    on the time slot margins. It should allow the network to stay    synchronized even if several beacons are not heard (i.e. it should    be smaller than MAC_Neighbor_Timeout). The more important the clock    drift is, the shorter the beacon period should be. The beacons are    not sent exactly at each period but are randomized in order to avoid    beacon collisions.    MAC_CELL_Leaving_Process_Duration:

Description: The time interval between the reception of a SYNC_ACK fromanother cell (considered better by the endpoint) and the moment theendpoint actually switches cell.

MAC_CELL_Switch_Hysteresis

-   Description: This parameter defines the hysteresis in the decision    process for cell switching.    MAC_CELL_Weight_CSI-   Description: In the cell switching process, this parameter defines    the weight of the cell size in cell selection.    MAC_CELL_Weight_GPD-   Description: In the cell switching process, this parameter defines    the weight of the GPD in cell selection.    MAC_CELL_Weight_Level-   Description: In the cell switching process, this parameter defines    the weight of the level in cell selection.    MAC_Clock_Accuracy.-   Description: This is the crystal accuracy defined to include all    influential parameters (tolerance, temperature and aging).-   Comment: The better the clock accuracy is, the easier the    synchronization is maintained.    MAC_Discovery_Beacon_Period:-   Description: The period between two discovery beacons sent during    the discovery phase.-   Comment: This should be larger than the time needed to send one    discovery beacon. It is also dependent on how quickly the    firmware/hardware can handle the transmission of a beacon.    MAC_Excellent_Connectivity_Threshold:-   Description: The minimum number of SYNC fathers from which a node is    considered to have an excellent connectivity degree.    MAC_FW_Accuracy:-   Description: The accuracy of the firmware for dating the actual    sending/reception of a message.-   Comment: This depends on the MCU and clock frequency.    MAC_GPD_TD:

Description: This parameter is used to model the fixed propagation delaythrough each node of the network. It is used to compute the GPD (Globalaverage Propagation Delay).

Comments: Increasing the value of this parameter will make the systemgive an advantage to the paths with fewer hops.

MAC_HFC_Max:

Description: Specifies the span of the hyperframe counter.

MAC_HF_Length:

-   Description: The length in time slots of a hyperframe.    A hyperframe is a succession of time slots that follows the hopping    hyper-sequence.-   Comment: This length is the product of the super-sequence length by    the number of channels.    HF_Length=Number_of_Channels*Hopping_Super_Sequence_Length    MAC_Hopping_Super_Sequence_Length:-   Description: The length of a frequency hopping super-sequence, also    the number of basic hopping sequences used in a hyper-sequence.    MAC_Listening_Window_Length:-   Description: Length of the listening window during the discovery    phase-   Comments: Increasing this length will decrease the probability of    collision between forced beacons but will slow down the discovery    process if several discovery phases are needed to find a SYNC    father.    MAC_LPD_Max:

Description: Maximum possible value for the LPD (Local averagePropagation Delay).

MAC_LPD_NAVG:

-   Description: The sliding average window length used to compute the    LPD (Local average Propagation Delay).-   Comment: this window must be smaller than the MAC_Neighbor_Timeout    value.    MAC_LPD_RSSI:

Description: Factor used to initialize the LPD (Local averagePropagation Delay) from the RSSI reading.

MAC_LPD_Switch:

Description: Factor used to initialize the LPD (Local averagePropagation Delay) from the RSSI reading.

MAC_Max_Discovery_Phase_Period:

-   Description: The maximum period between two discovery phases for a    non-synchronized endpoint.-   Comment: This is used after MAC_Max_Nb_of_Discovery_Phases    unsuccessful discovery phases and should be much larger than    MAC_Min_Discovery_Phase_Period.    MAC_Max_Nb_of_Discovery_Phases:-   Description: The maximum number of unsuccessful discovery phases    before increasing their period.

Comments: The reason for this parameter is to quiet down orphanedendpoints. It should be set high enough to make it possible for anendpoint to discover several cells.

MAC_Max_Nb_of Neighbors:

-   Description: The maximum size of the MAC Neighbor Table.    MAC_Max_Nb_of SYNC_Request:-   Description: The maximum number of times an endpoint tries to send a    SYNC request to each potential SYNC father.    MAC_Max_Nb_of_Transmitted_Bytes_sTS:-   Description: The maximum number of bytes that can be transmitted    during a sub time slot. This includes the entire packet, i.e., MPDU,    PHY header and preamble.-   Comment: This value, combined with the data rate, affects the Sub_TS    length.    MAC_Max_Nb_of Transmitted_Bytes_TS.-   Description: The maximum number of bytes that can be transmitted    during a time slot. This includes the entire packet, i.e., MPDU, PHY    header and preamble.-   Comment: This value depends on the data rate, the time slot length    and the time slot margins.    MAC_Max_Packet_Length.-   Description: The maximum length of PDU packets (expressed in bytes)    that the MAC layer authorizes the upper layer to send (the LPDU    length).-   Comment:    MAC_Max_Packet_Length=MAC_Max_Nb_of_Transmitted_Bytes_TS−(Preamble+PHY_header+FEC+MAC_Header+FCS).    MAC_Min_Discovery_Phase_Period:-   Description: The minimum period between two discovery phases for a    non-synchronized endpoint.-   Comment: This should be larger than the time needed to send the    discovery beacons plus    MAC_Listening_Window_Length.    MAC_Nb_of_Basic_Hopping_Sequences:-   Description: The number of basic frequency hopping sequences that an    endpoint can generate. Each hopping sequence is a succession of all    the predefined channels. Each one of the    MAC_Number_of_Channels channels appears once and only once in this    succession.-   Comment: This value should be greater than    MAC_Hopping_Super_Sequence_Length.    MAC_Nb_of_Sub_TS:-   Description: The number of sub time slots in a time slot. The    beginning of a Sub-TS marks the potential beginning of the    transmission of a packet.-   Comments: The number of Sub-TS is    ((TS_Length−2*TS_Margin)/Sub_TS_Length). It is assumed that the    length values are chosen to make this is an integer.    MAC_Max_nb_of downlink_buffered_packets:-   Description: The maximum number of packets that an endpoint can save    into its memory. It concerns only the packets going from the Cell    Master to the endpoints (Downlink).    MAC_Max_nb_of uplink_buffered_packets:-   Description: The maximum number of packets that an endpoint can save    into its memory. It concerns only the packets going from endpoints    to the Cell Master (Uplink).    MAC_Neighbor_Timeout:-   Description: The time limit for the MAC layer to decide that an    endpoint is not a neighbor anymore because the last reception    happened more than MAC_Neighbor_Timeout ago.-   Comments: This depends on the clock drift and the time slot margins.    An endpoint should not be considered a neighbor if there is a chance    it is no longer synchronized.    MAC_Number_of Channels:-   Description: The number of channels used in the basic frequency    hopping sequences.-   Comments: The minimum value for this parameter is fixed by the    spread spectrum regulations: 15, 20, 25 or 50 channels depending on    countries and smart frequency hopping features.    MAC_RSSI_Sampling_Rate:-   Description: Frequency of RSSI reads during the environment analysis    for the computation of averaged RSSI and RSSI auto-correlation    function.    MAC_RXI_Decay:-   Description: This constant is used to compute the Reception Rate    Indicator (RXI). The RXI is periodically multiplied by this constant    to make the indicator decay in time when no message is received.    MAC_RXI_Increment:-   Description: This constant is used to compute the Reception Rate    Indicator (RXI). Upon reception of a message from a neighbor, its    RXI is incremented by this constant.    MAC_RXI_Threshold:-   Description: RXI values above this threshold are considered to be an    indication of a reliable link. This is used in the computation of    figures of merit for the choice of synchronization fathers.    MAC_SACT_Resolution:-   Description: The value of the least significant bit of the SACT    timer when the value of this timer is exchanged between endpoints.-   Comment: The SACT timer can be locally implemented with a higher    resolution given by the parameter MAC_FW_Accuracy.    MAC_Sub_TS_Length:-   Description: The length of a Sub_TS. It corresponds to the maximum    length of the biggest MAC message (preamble+PHY header+FEC+MAC    header+FCS).-   Comment: It should be rounded up to obtain an integer number of    Sub_TS per TS.    MAC_SYNC_Disc_Weight_CSI-   Description: In the discovery phase, this parameter defines the    weight of the CSI of neighbors in the synchronization father    selection process.    MAC_SYNC_Disc_Weight_GPD-   Description: In the discovery phase, this parameter defines the    weight of the GPD of neighbors in the synchronization father    selection process.    MAC_SYNC_Disc_Weight_Level-   Description: In the discovery phase, this parameter defines the    weight of the level of neighbors in the synchronization father    selection process.    MAC_SYNC_Father_Request_Beacon_Threshold:-   Description: Duration used to determine if an endpoint is still in    synchronization with a father before accepting that another endpoint    gets synchronized with it. If a message from a healthy father has    been received within this time, there is no need to request a beacon    from it before answering a SYNC request.-   Comment: This value must be smaller than MAC_Beacon_Period_SYNC.    MAC_SYNC_Request_Period:-   Description: The minimum period between two different SYNC requests    sent to the same potential SYNC father.    MAC_Traffic_Density_max

Description: Any endpoint in the mesh will adjust its transmissionrandomization window in order to avoid creating a traffic input densityabove that limit for each one of its fathers.

Comments: The value of this parameter should always be less than one.Values close to one can improve the maximum throughput of the system butincrease the risk of jamming the data traffic with collisions.

MAC_Traffic_Monitoring_Window

Description: Length of the sliding average window used by an endpoint tomonitor the traffic of neighbors. This length is expressed in time slotsunits.

MAC_TS_Length:

-   Description: The length of a time slot. During the entire time slot,    the same channel will be set except for sending forced beacons.-   Comments: The maximum TS_Length value may be fixed by spread    spectrum regulations in some countries, (e.g., 400 ms for the    PHY-FHSS-NA-915). The default length corresponds to    ((Max_Nb_of_Tansmitted_Bytes_TS*8)/Bit_Rate)+2*TS_Margin.    MAC_TS_Margin:-   Description: The duration of each time interval at the extremities    of a time slot when an endpoint is not allowed to transmit. When in    receive mode, the endpoint should be listening across the whole time    slot, including these intervals, in order to be able to complete the    reception of a message coming from a neighbor with slightly    misaligned time slots.-   Comment: The TS_Margin value depends on the worst clock drift    between two endpoints and between two received messages. Wider    margins allow for more crystal drift or more time between messages    but decrease the maximum number of transmitted bytes per TS and thus    the throughput of the system.    MAC_Tx_Window_max:

Description: The maximum value for the randomization window used by anendpoint to transmit its data packets.

Comments: Only one packet should normally be transmitted in arandomization window and the position of the packet within this windowis random. The protocol does not forbid short packets to be transmittedin the same window but this must also follow the rules of priority.

MAC_Tx_Window_min:

Description: The minimum value for the randomization window used by anendpoint to transmit its data packets.

MAC_Unsynchronized_Timeout:

-   Description: Duration after which an endpoint still in discovery    phase will reset its notion of forbidden cell (and number of    discovery phases it already tried).    MAC_Warm_Start_Discovery_Duration:-   Description: Number of discovery phases during which an endpoint    with a preferred cell tries to synchronize only with it.-   Comments: This value should be large enough to ensure a high    probability of finding the same cell after an outage but small    enough to not delay the possible synchronization with another cell    if the preferred one is no longer available. This does not affect    the notion of forbidden cell.    MAC_Xdrift_Filter_A, MAC_Xdrift_Filter_B.-   Description: These are the filter coefficients for crystal drift    correction.-   Comments: Modifying these coefficients will make the filter step    response slower or faster. A faster step response can be desirable    to speed up the frequency synchronization of the nodes. Any    modification of these parameters should be done carefully to avoid    making the system unstable, see the relevant appendix.    MAC_Xdrift_LeapTS-   Description: This is the interval between leap time slots. Every    MAC_Xdrift_LeapTS time slots, the SACT is loaded with its initial    value plus a small correction to compensate for the drift of the    crystal.-   Comments: Large values of this parameter will increase the    resolution of the crystal drift compensation, but will also increase    the importance of the correction to be applied at each leap time    slot. Large values of MAC_Xdrift_LeapTS should only be used with    good crystals in order to avoid a desynchronization between two leap    time slots.    MAC_Xdrift_T_(min):-   Description: This is the minimum time interval over which the clock    corrections need to be summed before a new crystal drift correction    value can be computed.-   Comments: The value of this parameter depends on the average error    made when the clock (SACT) is adjusted from incoming packets. If    uncertainties in the time of arrival of packets are important, this    parameter should be increased to average out the uncertainties.

Depending on the local regulations and the frequency band in which theprotocol operates, the default values of the MAC parameters change. Thefollowing table gives default values for the parameters related to thegeneral or internal operation of the MAC layer as well as traffic loadmanagement.

AC- AC- MAC Parameters MAC-NA- NA- EU- MAC- MAC_(—) 915 2400 868 EU-2400Units Clock_Accuracy ±20 ppm FW_Accuracy 34 μs HFC_Max 255 hyperframesHF_Length 832 TS Hopping_Super_Sequence_Length 16 basic seqMax_Nb_of_Transmitted_Bytes_sTS 48 bytes Max_Nb_of_Transmitted_Bytes_TS288 bytes Max_nb_of_downlink_buffered_packets 5 packetsMax_nb_of_uplink_buffered_packets 5 packets Max_Packet_Length 176 bytesNb_of_Basic_Hopping_Sequences 16 sequences Nb_of_Sub_TS 6 sub-TSNumber_of_Channels 52 5 15 channels RSSI_Sampling_Rate 66.667 HzSACT_Resolution 100 μs Sub_TS_Length 20 ms Traffic_Density_max 0.8 —Traffic_Monitoring_Window 256 TS TS_Length 150 ms TS_Margin 15 msTx_Window_max 256 TS Tx_Window_min 10 TS Xdrift_Filter_A 1/16 —Xdrift_Filter_B 0.732 — Xdrift_LeapTS 256 TS Xdrift_Tmin 3 min

The next table gives default values for the parameters related to thediscovery phase.

MAC- MAC- MAC Parameters MAC-NA- NA- EU- MAC-EU- MAC_(—) 915 2400 8682400 Units Discovery_Beacon_Period 20 ms Listening_Window_Length 2.5 sMax_Discovery_Phase_Period 30 s Max_Nb_of_Discovery_Phases 156 PhasesMin_Discovery_Phase_Period 3.7 s Warm_Start_Discovery_Duration 104Phases

The next table gives default values for the parameters related tosynchronization, synchronization father choice, cell choice and neighbortable management.

MAC- MAC Parameters MAC-NA- MAC- MAC- EU- MAC_(—) 915 NA-2400 EU-8682400 Units Beacon_Period_SYNC 625 TS CELL_Leaving_Process_Duration 60seconds CELL_Switch_Hysteresis 400 — CELL_Weight_CSI 128 —CELL_Weight_GPD 1 — CELL_Weight_Level 0 —Excellent_Connectivity_Threshold 3 potential SYNC fathers GPD_TD 8 —LPD_NAVG 32 — LPD_Max 255 — LPD_Switch −70 dBm LPD_RSSI 3 —Max_Nb_of_Neighbors 10 neighbors Max_Nb_of_SYNC_Request 10 attemptsNeighbor_Timeout 10 minutes RSSI_Var 225 — RXI_Decay 0.9 — RXI_Increment20 — RXI_Threshold 80 — SP_CD1 100 — SP_LPD1 200 — SP_LPD2 1000 —SP_LPD3 4000 — SP_RXI1 400 — SP_RXI2 2000 — SYNC_Disc_Weight_CSI 128 —SYNC_Disc_Weight_GPD 1 — SYNC_Disc_Weight_Level 50 —SYNC_Father_Request_Beacon_Threshold 625 TS SYNC_Merit_Hyst1 50 —SYNC_Merit_Hyst2 150 — SYNC_Request_Period 10 TS SYNC_Top_N 10 —Unsynchronized_Timeout 2 days

The following generally discusses frequency and time division facets ofthe present subject matter. In particular, the present protocol subjectmatter is based on a frequency hopping system for better interferenceimmunity and to be in accordance with the spread spectrum regulations insome countries. In North America, a frequency hopping system allows atransmitted power higher than a system using a single narrow channel. Itmeans that frequency and time will be divided. To establish an RF linkbetween two nodes, two conditions have to be respected, which are thatthe two entities have to operate at the same frequency and at the sametime. The protocol respects these two conditions by adopting a time slotscheme.

The time frame is divided per present subject matter into regular timeslots, each one operating on a different frequency. In the UnitedStates, the FCC rules (Part 15.247) specify that for a FHSS system, theaverage time of occupancy on any frequency shall not be greater than 0.4second in a 20-second window. In Europe, the same limitation applies inthe 2.4 GHz band. On the other hand, there is no dwell time per channellimitation in the 868 MHz band. Devices operating in the 868 MHz bandshall comply with a general duty cycle limit that is averaged over allthe channels used by the system. For the present subject matter, theduration of a time slot has been sized to contain one singlemaximum-size message, as represented by present FIG. 11.

Applicable FHSS regulations also specify the minimum number offrequencies that have to be used. In North America, in the 915 MHz band,if the channels are less than 250 kHz wide, the minimum number ofchannels is 50. For the 2.4 GHz band, in North America as well as inEurope, at least 15 channels are required. There is an exception to suchrules. In particular, in the 2.4 GHz European band, at least 20 channelsare required if an adaptive frequency hopping strategy is used. Adaptivefrequency hopping allows the selection of the best channels to avoidinterference. On the other hand, there is no minimum number of channelsin the 868 MHz European band. The more channels there are in the system,the higher the isolation between adjacent cells will be, but the longerwill be the synchronization time.

In order to make an RF communication possible, any two nodes of thenetwork need to know precisely which channel to use in every time slot.To make such operation possible, the channel use is organized accordingto a frequency hopping sequence known by all the endpoints belonging tothe network. Such sequence is designed to use all the channels equallyon average.

To provide isolation between adjacent cells, each cell has its ownfrequency hopping sequence. Such sequence per present subject matter isdiscovered by all the endpoints of the cell during the synchronizationprocess. To add more isolation between the cells, it has been chosen perpresent subject matter to organize the hopping pattern into hyperframes.See also present FIG. 12 representing present hyperframe and channelsequence subject matter, based on 15 exemplary channels, with 10 basicsequences. Such a hyperframe technique follows a frequency hoppinghyper-sequence built with several different basic frequency hoppingsequences, which make a longer sequence but always with the same sub-setof channels. The sequence is now constituted with K different basicsequences, which means MAC_HF_Length=K*MAC_Number_of_Channels timeslots. Such present approach also adds a better immunity to jammers. Ina given cell, all the hyperframes are preferably identical per presentsubject matter. Once the system has swept across a hyperframe, itrepeats the same sequence in a periodic way.

With respect to present channel sequence assignment, per the presentsubject matter, all the endpoints know the different channel sequencesthat can be used but only one channel sequence is used by cell. Thechannel sequence is given by the cell relay. The sequence is computed bythe knowledge of the specific algorithm which uses the C12.22 address ofthe cell relay as seed. To change the hopping sequence in a cell, thecollection engine has to change the address of the cell relay.

If there are several cell relays in the same area, it is imperative thatthey follow different channel sequences. Only one cell relay is possibleby cell (because cell relays are not synchronized with each other). Onthe contrary, per present subject matter, a high number of differentcells can operate in the same area because they don't hear each othermost of the time.

The cell number given to the cell relay is transmitted and forwarded toall the endpoints in the cell, per present subject matter. Such field isused to generate the hopping sequence, as otherwise described herein.

Cell isolation is achieved per present subject matter preferably throughquasi-orthogonal sequences in a frequency hopping network. Moreparticularly, in accordance with present subject matter, two nestedpseudo-random sequences are utilized to isolate cells in a frequencyhopping spread spectrum network. The inner sequence is generated withGalois field arithmetic and the outer sequence is a Fibonacci sequencethat uses the cell unique address as a seed.

The present protocol subject mater is based on frequency hopping spreadspectrum (FHSS) for better interference immunity and compliance withradio regulations in some countries. In a typical FHSS system all thenodes hop their channel frequency according to the same pseudo-randomsequence in order to be perfectly synchronized for both reception andtransmission. The frequency hopping pattern is the rule that assigns achannel number to each time slot of the slotted Aloha protocol. Suchpattern is periodic, and it will repeat itself indefinitely.

As it is very difficult to maintain a good synchronization for a verylarge number of nodes, the system of the present subject matter isadvantageously divided into cells. Each cell contains a limited numberof nodes and has its own pseudo-random sequence for transceiversynchronization. Per present subject matter, all the nodes inside a cellare synchronized with each other but the different cells are notsynchronized with each other. Technical problems of such approach whichare addressed per the present subject matter include how to find asimple way to generate a pseudo-random sequence for each cell, and howto make sure that these sequences will be unique and provide sufficientisolation between adjacent cells.

The present subject matter combines the use of Galois field arithmeticand Fibonacci sequences to generate pseudo-random sequences. Theresulting sequence is the combination of two nested sequences. The innerone is generated by Galois field arithmetic and the outer one isgenerated by a Fibonacci sequence using the cell address as a seed. Thecell address is unique to each cell and will lead to completelydifferent sequences for any two adjacent cells even if the addressesdiffer only by one.

The following is a formal description of the present hopping patternconstruction subject matter.

The inner hopping sequence is constructed with a Galois field of orderp, where p is a prime number. Addition and multiplication in this fieldare to be carried out modulo p. This Galois field isGF(p)={0, 1, 2, 3, . . . , p−1}

In any Galois field, one can find primitive elements. An element is saidto be primitive if its successive powers generate all the non-zeroelements of the field. If α is a primitive element of the field, thefield can be written as follows:GF(p)={0, 1, α, α², α³, . . . α^(p−2)}

To create the inner hopping sequence from this, only the non-zeroelements of the field are selected and definition as follows isestablished of the ordered (p−1)-tuple (1, α, α², α³, . . . α^(p−2)).The length of the inner hopping sequence (the number of hops in thesequence), is equal to the number of channels used by the system, N=p−1.The RF link will use several different inner hopping sequences. Each oneof such sequences will be generated by its own primitive element. Kdifferent primitive elements are selected to generate K different innerhopping sequences. Such primitive elements are (α₀, α₁, α₂, . . .α_(k−1)).

The inner hopping sequences are numbered from 0 to K−1. The k-th hoppingsequence is generated by the k-th primitive element and its expressionis:IHS(k)=(1, α_(k), α_(k) ², α_(k) ³, . . . α_(k) ^(N−1))Or, in a compact formIHS(k,n)=α_(k) ^(n) , k=0, 1, 2, . . . K−1, n=0, 1, 2, . . . N−1

Where IHS (k,n) is the channel number of the n-th hop of the k-th innerhopping sequence and α_(k) is the primitive element of the k-th innerhopping sequence.

The outer hopping sequence is intended per present subject matter toprovide a hopping pattern which is unique to each cell. To make thepattern unique, the outer hopping sequence is built with a Fibonaccisequence using the cell identifier as a seed, as follows:OHS(0)=Cell Identifier(0)OHS(1)=Cell Identifier(1)OHS(n)=OHS(n−1)+OHS(n−2), n=2, 3, 4, . . . , M−1

Here the cell identifier is broken up into two parts: the mostsignificant and the least significant part. The integer M is the lengthof the outer sequence As a present alternative, any extension of thisprocess is possible per present subject matter. For example, one canbreak up the cell identifier in four parts and use a Tetranaccisequence, as follows:OHS(n)=Cell Identifier(n), n=0, 1, 2, 3OHS(n)=OHS(n−1)+OHS(n−2)+OHS(n−3)+OHS(n−4), n=4, . . . , M−1

The elements of the outer sequence are used to select a specific N-hopinner frequency hopping sequence. For such purpose, the Fibonaccisequence elements are computed with modulo arithmetic in order to mapinto the set of available hopping sequences. From the inner sequence andthe outer sequence the resulting hopping hyper sequence is obtained withthe following nesting procedure:

{[IHS(OHS(0), 0), IHS(OHS(0), 1), …  , IHS(OHS(0), N − 1)], [IHS(OHS(1), 0), IHS(OHS(1), 1), …  , IHS(OHS(1), N − 1)], …  [IHS(OHS(M − 1), 0), IHS(OHS(M − 1), 1), …  , IHS(OHS(M − 1), N − 1)]}

As the computation of high powers of a primitive element can bedifficult to implement on a microprocessor, it is recommended perpresent subject matter to compute the inner hopping sequenceiteratively. With the first hop always designated as 1, each successivehop can be easily computed from the preceding hop by the followingequation.

$\left\{ {\begin{matrix}{{{IHS}\;\left( {k,0} \right)} = \; 1} & {n = 0} \\{{{{{IHS}\;\left( {k,n} \right)}\; = \;{\alpha_{k}\;{IHS}\;\left( {k,{n - 1}} \right)}},}\mspace{11mu}} & \begin{matrix}{{{k = 0},1,2,\ldots\mspace{14mu},{K - 1},}\mspace{14mu}} \\{{n = 1},2,{{\ldots\mspace{14mu} N} - 1}}\end{matrix}\end{matrix}\quad} \right.$

This inner hopping sequence is very readily computed with only theknowledge of the previous hop and the hopping sequence number. This isintended per present subject matter to be computed before each hop,avoiding the need to store all the possible hopping sequences in memory.

The use of two nested sequences per the present subject matteradvantageously improves spreading and interference immunity. Also, asimple, iterative way to compute the hopping sequences is provided, asimple way to generate hopping sequences unique to each cell isprovided, a simple way to isolate the cells is provided, and theiterative implementation leads to very low memory and computation loadrequirements.

The hopping super-sequence of the present subject matter is intended toprovide a hopping pattern which is unique to each cell. To make thepattern unique, the hopping super-sequence is built with a Fibonaccisequence using the cell identifier CELL as a seed. CELL is a 2-byteaddress. That is first broken up into four 4-bit numbers, as follows:

$\left\{ {\begin{matrix}{{{CELL}\left( {15,14,13,12} \right)} = {C_{id}(0)}} \\{{{CELL}\left( {11,10,9,8} \right)} = {C_{id}(1)}} \\{{{CELL}\left( {7,6,5,4} \right)} = {C_{id}(2)}} \\{{{CELL}\left( {3,2,1,0} \right)} = {C_{id}(3)}}\end{matrix}\quad} \right.$

The Fibonacci sequence is then constructed with the following:

C_(id)(n) = C_(id)(n − 1) + C_(id)(n − 2) + C_(id)(n − 3) + C_(id)(n − 4), n = 4, 5, …  , M + 3

In that sum all the additions should be carried out modulo 16. Each termin the sequence is then an integer between 0 and 15, as follows:0≦C _(id)(n)≦15

The integer M is the length of the super-sequence. One of ordinary skillin the art will observe that this particular indication is not a bonafide Fibonacci sequence because four terms are used in the sum insteadof two. Some authors have coined the word Tetranacci to name such asequence. The hopping super-sequence will be:HSS=(C _(id)(4), C _(id)(5), . . . , C _(id)(M+3))

This can also be written:HSS(n)=C _(id)(n+4), 0≦n≦M−1

The elements of the super-sequence are used to select a specific N-hopbasic frequency hopping sequence. If K=16, each element of the Fibonaccisequence points naturally to a hopping sequence. This is the case forthe PHY-FHSS-NA-915 physical layer. If less than 16 basic hoppingsequences are available, The Fibonacci sequence elements are convertedto modulo K integers in order to map into the set of available hoppingsequences, as follows:

HSS(n) = MOD(C_(id)(n + 4), K), 0 ≤ n ≤ M − 1

Given the hopping super-sequence:HSS=(HSS(0), HSS(1), . . . HSS(M−1))And the set of K basic hopping sequences:HS(k)=(HS(k,0), HS(k,1), . . . HS(k,N−1)), 0≦k≦K−1The hyper-sequence is constructed in the following way:

{[HS(HSS(0), 0), HS(HSS(0), 1), …  , HS(HSS(0), N − 1)], [HS(HSS(1), 0), HS(HSS(1), 1), …  , HS(HSS(1), N − 1)], …[HS(HSS(M − 1), 0),  HS(HSS(M − 1), 1), …  , HS(HSS(M − 1), N − 1)]}

This M*N-hop hyper-sequence repeats itself indefinitely in a periodicway. During the first time slot, the endpoint's transceiver will use thefrequency indicated by the first element of this hyper-sequence for bothtransmission and reception activities. For the second time slot, it willuse the second element and so on.

The length of the hyper-sequence, M*N, is related to the MAC layerparameter:M*N=MAC_HF_Length

A special case is worth mentioning. If the cell identifier is empty,that is, if it contains only zeros, the super-sequence will be aconstant sequence of zeros. In this case the hyper-sequence reduces tothe repetition of the first basic hopping sequence as follows:[HS(0,0), HS(0,1), . . . , HS(0, N−1)]

The succession of M*N time slots using the channels given by thishyper-sequence is called a hyperframe. Present FIG. 13 illustrates apresent exemplary structure of a hyperframe. The basic hopping sequenceindex is the hop number within each basic hopping sequence and thesuper-sequence index specifies the position within the super-sequence.

The hyper-sequence of the present subject matter has been designed toavoid having two different cells working with the same hopping pattern,provided they have different cell identifiers as defined herein. As itis likely that two adjacent cells will have close cell identifiers, ithas been checked that the proposed algorithm leads to two very differenthopping patterns even if the cell identifiers differ by only one bit.

These sequences are nevertheless not totally orthogonal and somecollisions are unavoidable. One should also keep in mind that the clocksof adjacent cells will drift with respect to each other. The consequenceis that two super-sequences that differ only by a cyclic permutationwill not necessarily provide cell isolation in a reliable way. Theprobability for this to happen is however believed to be very low.

For the physical layer of the exemplary embodiment PHY-FHSS-NA-915(regarding 915 MHz), the number of channels is:N=MAC_Number_of_Channels=52The number of basic hopping sequences is K=16, the super-sequence lengthis M=16 and the hyper-sequence length is:M*N=MAC_HF_Length=52×16=832Present FIG. 14 gives the primitive elements for the K basic hoppingsequences for a PHY-FHSS-NA-915 exemplary embodiment.

The basic hopping sequence generation rule is, for each of the 16sequences

$\left\{ {\begin{matrix}{{{HS}\;\left( {k,0} \right)} = 1} & {n = 0} \\{{{{{HS}\;\left( {k,n} \right)} = {{MOD}\left( {{\alpha_{k}\;{HS}\;\left( {k,{n - 1}} \right)},53} \right)}},}\mspace{11mu}} & \begin{matrix}{{{k = 0},1,2,\ldots\mspace{14mu},15,}\mspace{14mu}} \\{{n = 1},2,{\ldots\mspace{14mu} 51}}\end{matrix}\end{matrix}\quad} \right.$

As an example, this is the detail for generation of the basic hoppingsequence number 2. From the table of FIG. 14, the primitive element forthe basic hopping sequence number 2 is 5. The sequence will be computedby:

$\left\{ {\begin{matrix}{{{HS}\;\left( {2,0} \right)} = \; 1} & {n = 0} \\{{{{{HS}\;\left( {2,n} \right)} = {{MOD}\left( {{5^{*}\;{HS}\;\left( {2,{n - 1}} \right)},53} \right)}},}\mspace{11mu}} & {{n = 1},2,{\ldots\mspace{14mu} 51}}\end{matrix}\quad} \right.$

The first hop is always:HS(2,0)=1(channel number 1 will be used)

The second hop is:HS(2,1)=modulo(5*1;53)=5(channel number 5 used as example)

The third hop is:HS(2,2)=modulo(5*5;53)=25(channel number 25 used as example)

The fourth hop is:HS(2,3)=modulo(5*25;53)=modulo(125;53)=125−2*53=19

The fifth hop is:HS(2,4)=modulo(5*19;53)=modulo(95;53)=95−53=42

The sixth hop is:HS(2,5)=modulo(5*42;53)=modulo(210;53)=210−3*53=51

Such process continues until the 52 hops are computed.

With reference to conducting communications with an adjacent cell,whether the hopping sequence should be implemented by a modulo nmultiplication or by a table look-up is a question of trade-off betweencomputation speed and memory. Although the iterative approach has beensuggested above, either choice may be made by the user, in accordancewith the present subject matter, without adversely affecting the broaderaspects of the present protocol subject matter.

There is a situation where the conditions of such trade-off aredifferent. When an endpoint wants to communicate with another endpointbelonging to a different cell, there is a need for it to be able toquickly compute the hopping pattern of the new cell in order to be ableto chime in with the right frequency in the middle of the hoppingpattern. If the iterative multiplication process is used in such case,it would lead to a number of modulo p multiplications as large as thehop number in the basic sequence. If this is in a given instance anexcessive burden for the microcontroller, the present subject matter mayalternatively use the exponentiation by squaring algorithm to speed upthe computation. Such algorithm, adapted from cryptographiccomputations, can be applied iteratively and will lead to a number ofoperations proportional to the logarithm in base two of the hop number.The gain on computation time is therefore relatively large.

The exponentiation by squaring algorithm consists in the iterativeapplication of the following equation:

$x^{n} = \left\{ \begin{matrix}x & {n = 1} \\\left( x^{2} \right)^{\frac{n}{2}} & {n\mspace{14mu}{even}} \\{x\left( x^{2} \right)}^{\frac{n - 1}{2}} & \begin{matrix}{n\mspace{14mu}{odd}} \\{n > 1}\end{matrix}\end{matrix} \right.$This algorithm will be used to compute the channel frequency startingfrom the following expression of the hopping sequence:HS(k,n)=α_(k) ^(n)

The following example is based on computing the channel number for thehop number 33 of the basic sequence number 7 of the PHY-FHSS-NA-915physical layer.

From the above-referenced table (FIG. 14), the basic hopping sequencenumber 7 uses the primitive element 19. Therefore, is computed:HS(7,33)=α₇ ³³=19³³

A first application of the algorithm leads to:HS(7,33)=19×19³²=19(19²)¹⁶(mod 53)

Now performing the first squaring: 19²=361=43(mod 53) and on to the nextstep:HS(7,33)=19(43)¹⁶=19(43²)⁸(mod 53)

Second squaring: 43²=1849=47(mod 53)HS(7,33)=19(47)⁸=19(47²)⁴(mod 53)

Third squaring: 47²=2209=36(mod 53)HS(7,33)=19(36)⁴=19(36²)²(mod 53)

Fourth squaring: 36²=1296=24(mod 53)HS(7,33)=19(24)²(mod 53)

Fifth squaring: 24²=576=46(mod 53)

And a final multiplication:HS(7,33)=19×46=874=26(mod 53)

Even though the channel number is 26, the subject exemplary computationhas been computed in only 6 operations instead of 32.

With reference to message positions and sub time slots, messages willhave very different lengths. At one extremity, one would find MACmessages, such as beacons, which occupy a small percentage of the timeslot duration, and at the other extremity, one would find messages fromabove layers, which occupy a complete time slot.

The TS length has been sized to contain one message of the maximum size,MAC_Max_Nb_of_Transmitted_Bytes. If the API layer requests a longermessage, the LLC will fragment this message in packets. Of course, eachpacket will not exceed MAC_Max_Nb_of_Transmitted_Bytes (PHY header andpreamble included), as otherwise also referenced in conjunction with theLLC layer description.

The MAC messages are the shortest messages that can be sent. Since thephysical layer is by default in receiving mode, packets can be receivedin a timeslot where a packet has been sent. To limit the number ofcollisions inside a TS between the received and transmitted packets,timeslots are divided in sub-timeslots (Sub_TS) of equal sizes(MAC_Sub_TS_Length). Their size is set to fit in the longest of the MACmessages. For example, in FIG. 15 (showing TS margins and Sub TimeSlots), up to 6 beacons from different neighbors can be received in asingle TS.

The maximum size of a message that can fit into a sub time slot isMax_Nb_of_Transmitted_Bytes_sTS. If a message is too long to fit in onesub time slot, it will use several, but it will always start at thebeginning of a sub time slot in order to occupy a minimum number ofthem. This is actually the concept of slotted aloha access which is hereapplied to the sub time slot structure of a time slot.

When a message should be sent, the selection of the Sub_TS number israndomized between the second and last sub-TS for a MAC message; A LPDUmessage will always start at the beginning of a TS (so in the firstsub-TS).

Another aspect in the division of a TS, is the TS margins. At thebeginning and at the end of each TS, a part of it has to be free oftransmissions. If transmissions are not allowed in this part of the TS,receptions are.

These margins give the possibility of communication between twoendpoints that are not perfectly synchronized. Length of these margins,MAC_TS_Margin, reflects the maximum authorized error of timesynchronization between two resynchronizations, in the worst-casescenario (maximum clock errors, several missed beacons, no traffic,etc.).

The network per the present subject matter is divided into cells, wherethe traffic is expected to be low. Moreover, the network operates in anISM band, where many other users occupy the same medium (with the samerules). So, the probability of collision due to the external environmentis likely to be higher than the probability of collision within thesubject network itself. That's why the slotted Aloha algorithm isappropriate to manage the access to the medium. The entire cell issynchronized in time and frequency (as described herein). When anendpoint wants to speak, it just pushes its message onto the medium on arandom time slot. The recipient will receive the message because it issynchronized and because by default, an endpoint is listening to themedium (Physical Layer is, by default, in receive mode). Obviously, acollision occurs when two close endpoints want to speak in the same timeslot, but this is solved by a repetition of the messages, repetitionmanaged by the LLC layer. When pushing a data on the medium, the MAClayer doesn't care if it is an Uplink or Downlink message; the bit rateand all other parameters are the same for both ways. Data transmissionis non-hierarchical and symmetrical.

Because channels are equally represented and because data can be pushedonto any time slot, the present protocol subject matter respects theuniform occupancy of the media over the band.

It is very important that the traffic remains very low to ensure a goodworking of the Slotted Aloha access. Slotted Aloha would allow up to 35%data load if the subject network were alone on the media. In a real-lifesituation, 3% data load is more adequate.

Each time a packet is received from a neighbor, the physical layer makesavailable an RSSI reading for that packet. For each neighbor in itsneighbor table, the MAC layer will compute an average value of this RSSIwith a Kalman filter. This filter will give an accurate estimate of theaverage RSSI as soon as a few RSSI readings will be available. Thefollowing pseudo-code gives the details of this algorithm:

Upon reception of a packet from neighbor X If this is the first packetreceived from X, then RSSI_Average = current RSSI reading for thatpacket RSSI_Cov = 255 Else compute the new RSSI_Average with${KG} = \frac{\left( {{Old}\mspace{14mu}{RSSI\_ Cov}} \right)}{\left( {{Old}\mspace{14mu}{RSSI\_ Cov}} \right) + {{MAC\_ RSSI}{\_ Var}}}$New RSSI_Average = (1 − KG)(Old RSSI_Average) + KG(Current RSSI reading)New RSSI_Cov = (1 − KG)(Old RSSI_Cov) End if

Update the neighbor table with the new RSSI_Average value and the newRSSI_Cov value.

RSSI_Cov is the estimate of the covariance of the RSSI, it has to bekept in memory for each neighbor as well as the average RSSI,RSSI_Average. RSSI_Var is a MAC layer parameter that corresponds to anestimate of the RSSI variance. RSSI and RSSI_Average are 1-byte two'scomplement coded variables. Their range extends from −128 dBm to +127dBm. RSSI_Cov is a I-byte positive variable. KG is the Kalman gain, itis an intermediate result in the Kalman filter computation and its valueis always less than one.

The Average RSSI gives a fair indication of the received signal quality,even in environments plagued with Rayleigh fading. As explained inanother section of this document, this Average RSSI participates in thechoice of the best synchronization father.

It is the task of the MAC layer to update the LPD (Local averagePropagation Delay) associated with each neighbor and to compute the GPD(Global average Propagation Delay) from the endpoint to the Cell Relayvia each neighbor. These values are used to sort and select neighbors.They are used to select the best access for synchronization or to make achoice between different available cells. The network layer will usethese values to choose the best path for uplink (routing packets).

After every packet transmission that requires an acknowledgement, theMAC layer will know if the packet transmission was successful or not. Ifa positive or negative acknowledgement is received, the transmissionwill be considered successful. If no acknowledgement is received, thetransmission will be considered to have failed. For every neighbor inits neighbor list, the MAC layer will update the individual LPD valuesas shown below:

${{New}\mspace{14mu}{LPD}} = \left\{ \begin{matrix}\frac{{{MAC\_ LPD}{\_ NAVG} \times {Old}\mspace{14mu}{LPD}} + 16}{{{MAC\_ LPD}{\_ NAVG}} - 1} & {{if}\mspace{14mu}{transmission}\mspace{14mu}{failed}} \\\frac{16\left( {{{MAC\_ LPD}{\_ NAVG}} - 1} \right) \times {Old}\mspace{14mu}{LPD}}{{{Old}\mspace{14mu}{LPD}} + {16 \times {MAC\_ LPD}{\_ NAVG}}} & {{if}\mspace{14mu}{transmission}\mspace{14mu}{succeeded}}\end{matrix} \right.$

In these equations the MAC parameter MAC_LPD_NAVG is an integer valuethat determines the depth of the sliding average window. A scalingfactor of 16 has been introduced to allow an integer representation ofLPD. The maximum allowed value for LPD is LPD_Max, any LPD computedvalue above LPD_Max should be truncated to LPD_Max. Note 1: LPD is aninteger and when the computation gives a number with decimals, thesedecimals should be truncated. Note2: the new LPD must always bedifferent from the old one (except when the values are zero or LPD_Max).If the new LPD equals the old one and there was a failure the new LPDshould be incremented by one; if there was a success of transmissionthen the LPD should be decremented by one.

The GPD (Global average Propagation Delay) is the average propagationdelay between the endpoint and its associated Cell Relay. The networkwill compute this value step by step from the Cell Relay down to theendpoint. To avoid confusion, one can consider the following notation:

EP_GPD(X)≡Global Propagation Delay between the endpoint and the CellMaster if traffic is routed through the neighbor X.

An endpoint can compute the GPD to the Cell Relay via each one of itsneighbors according to the following equation:EP_GPD(X)=GPD of neighbor X+LPD between endpoint and neighborX+MAC_GPD_TD

MAC_GPD_TD is a MAC layer parameter introduced to model the fixedpropagation delay through each node of the network (see appendix on pathselection algorithm). The best one of these values will be called theendpoint GPD.GPD=Min{EP_GPD(X)}for all neighbors X that are registered fathers

This GPD value will be included in the MAC header to make thisinformation available to other endpoints. The allowed values for GPD arethe integers between zero and 4095.

The node should update its GPD when it changes level or switches toanother cell. The endpoint needs also to check if its GPD has notchanged at each reception of a message from one of its fathers. In amore general way, an endpoint should always keep the lowest GPD it can,among its registered fathers (from the same cell).

At start-up the LPD values in the neighbor list should be initializedaccording to the RSSI value of the first message received from theseneighbors. The initialization follows this rule:

${LPD} = \left\{ \begin{matrix}0 & {{{if}\mspace{14mu}{RSSI}} \geq {LPD\_ Switch}} \\{{Min}\left\lbrack {{{- \left( {{RSSI} - {LPD\_ Switch}} \right)}{LPD\_ RSSI}},{LPD\_ Max}} \right\rbrack} & {{{if}\mspace{14mu}{RSSI}} < {LPD\_ Switch}}\end{matrix} \right.$

where LPD_RSSI and LPD_Switch are MAC layer parameters.

Stated another way, the present protocol subject matter providesadvantageously for uplink routing without requiring a routing table.Such is achieved by addressing the main uplink path in the subjectnetwork. Such communication is used to convey the data, from every nodeof the network to a single extraction point. The challenge associatedwith such feature and presently achieved is for a node to find the pathtowards the extraction point without the knowledge of the networktopology, that is, without a routing table. Following the objective ofreaching the extraction point in a shortest time, the traffic shouldalso be relatively spread so as to avoid peaks, as the traffic becomesmore dense close to the extraction point.

Conceptually, per present subject matter, the synchronization processhas given a level to every node in the network. Such level representsthe number of hops between the node and the extraction point. Each nodehas a certain number of neighbor with a lower level (closer toextraction point), called fathers (or, parents) of the node; equallevel, called brothers; and higher level (further from extraction point)called sons.

In accordance with present subject matter, a node should preferablypropagate an uplink message to one of its fathers, which means a closernode from the head of the network. The message will at the end convergeon the extraction point. The selected father to route an uplink messageis belonging to the best father list. Fathers belonging to such list arethose with the best GPD. Computation of the Global Propagation Delay,GPD, is otherwise explained herein. The lowest GPD indicates theshortest path in time. The selected father is not necessarily always theone with the best GPD. The node sends uplink messages randomly to one ofthese best fathers with a probability for each father inverselyproportional to its GPD.

Advantageously, practice of such aspects of the present subject matterachieves the benefits that the shortest paths are selected, knowledgeconcerning only one hop neighbors is sufficient for the achievement,nodes don't need knowledge of the entire network, so that there are norouting table in the nodes, which results in a relatively large savingson required memory. In addition, generally speaking, traffic is spreadover the network, due to randomization between fathers.

One aspect of the present protocol subject matter advantageouslyprovides for real time clock distribution and recovery, particularlyapplicable in a network, for example, otherwise based on the slottedAloha protocol.

Generally speaking, time is presently divided into time slots (TS) andthe nodes send packets inside these time slots. The frequency used forcommunication changes at each TS according to a determined pattern: thehyperframe. A number, the Time Slot Number (TSN), is incremented at eachTS and it rollovers when it reaches the hyperframe length, at whichpoint the frequency pattern repeats itself. A second number, theHyperFrame Number (HFN), is also associated and incremented with eachhyperframe.

Nodes are grouped in cell around a concentrator (or root node) and these2 numbers are common to all the nodes in the cell; this way theirtransmitters are always set on the same RF frequency. These 2 numberswill be used by the nodes to refresh their “real-time” clock uponreceiving a specific message which originates from the root node.Effectively the distribution of the clock will be done from the rootnode (extraction point of the nodes' data), which is connected to theinternet and thus has access to an accurate real time clock (forexample, NTP standard).

Generally, nodes operate with crystals, which results in limitedaccuracy. The present challenge which is successfully addressed herewithis to refresh periodically the time in each node before its clock driftsbeyond the desired accuracy. Since the propagation is not instantaneousthe system has to take the propagation delay into account.

The present solution is to advantageously broadcast regularly atimestamp (RITP) provided by the chosen real time clock. The creation ofthe timestamp will always be done at the root node level when the TSNand HFN of the cell are both zero. This broadcast will also contain thehyperframe number (HFN) corresponding to the initial broadcast by theroot node; this allows detection of an overflow of HFN and adapt theRITP value in consequence. This message, following the broadcastprotocol, will reach all nodes in the cell; the HFN maximum value isdesigned for the rollover to be much over the maximum propagation delayof this real time clock distribution.

When a node receives this broadcast it can update its “real-time” clockusing the following formula:Absolute time=(TSN+HFN*hyperframe_length)*Timeslot_Length+RITPWhere the hyperframe length is expressed in number of timeslots and thetimeslot length is a time unit (note 150 ms in the present exemplarycase). Note: if the HFN of reception is lower than the HFN included inthe broadcast then there was a rollover and the RITP timestamp must beupdated by adding HFN rollover value*hyperframe_length*Timeslot_Length.Such present solution gives an absolute time with a resolution equal tothe time slot length.

When a node just synchronizes in a new network, the RITP timestamp (andHFN corresponding) is given in the synchronization acknowledgement. Thisway the new node has access to real time without waiting for the nextITP broadcast. Note: This assumes that the RITP broadcast is done eachtime HFN=TSN=0 to avoid more than one rollover of the HFN number.

Such aspects of the present protocol subject matter advantageouslyprovides a simple way, even while using slotted Aloha architecture, todistribute a real time clock to all the nodes with a resolution of oneTime Slot (despite the delay of propagation). It also allows a fastrecovery of the real time clock immediately upon synchronizing to a newnetwork.

Therefore, per present subject matter, time management in the MAC layeris performed with the use of several counters. FIG. 16 illustrates theglobal structure of such present protocol time keeping structurecounters, while the following provides some additional description foreach one of such counters.

As to the Slotted Aloha Countdown Timer (SACT), at the beginning of eachtime slot such timer is loaded with a value corresponding to the defaulttime slot length, MAC_TS_Length. When this timer reaches the value zero,a new time slot begins. Every MAC_Xdrift_LeapTS time slots, the SACT isloaded with the value MAC_TS_Length plus a small correction tocompensate for the drift of the crystal (see description of leap timeslots in the chapter on drift correction).

The SACT is locally implemented with a resolution specified by theparameter MAC_FW_Accuracy but when SACT values are exchanged betweenendpoints or included in the MAC header for synchronization purpose, theSACT is defined with a resolution of MAC_SACT_Resolution μs.

The content of the Time Slot Counter is the time slot number (TSN). Atthe beginning of each time slot this counter is incremented. Its valueruns from zero to MAC_HF_Length −1. MAC_HF_Length is the number of timeslots in a hyperframe.

The content of the Hyperframe Counter is the hyperframe number (HFN). Atthe beginning of each hyperframe this counter is incremented. Its valueruns from zero to MAC_HFC_Max −1. MAC_HFC_Max is the number ofhyperframes spanned by the MAC Clock.

The Time Slot Counter can be split in two cascaded counters, the BasicHopping Sequence Counter and the Super-Sequence Counter. The content ofthe Basic Hopping Sequence Counter indicates the position within a basichopping sequence. At the beginning of each time slot this counter isincremented. Its value runs from zero to MAC_Number_of_Channels−1.MAC_Number_of_Channels is the number of channels used in a basic hoppingsequence. The content of the Super-Sequence Counter indicates theposition within the hopping super-sequence. When a basic hoppingsequence is completed, this counter is incremented. Its value runs fromzero to (MAC_HF_Length/MAC_Number_of Channels−1).

The Relative ITP timestamp (RITP) is initialized to zero at start-up.Once absolute time information is obtained from the network or from theapplication, this counter can be updated. At each overflow of thehyperframe counter the Relative ITP timestamp must be updated asotherwise explained herein. This timestamp can be implemented with thestandard NTP (Network Time Protocol) format or with a shortened versionof the standard NTP format according to the required accuracy and span.

Present FIG. 17 generally represents a standard ITP (Itron TimeProtocol) timestamp format. From the above-referenced present protocoltime management structure, one can define several time values. Two whichare given here, will be useful for several purposes, and they are theabsolute time and the relative time.

The absolute time corresponds to the real time clock of the application.It can be used to date any event in an absolute way. Its resolution isthe time slot length. In terms of the time keeping counters, theabsolute time value is given by the formula already referencedhereinabove.

In contrast, the relative time is used to measure durations on a timescale smaller than the span of the MAC clock. This time has a higherresolution because it uses the SACT. In terms of the time keepingcounters, the relative time value is given by:Relative time=(TS_Length−SACT*“SACT timeunits”)+(TSN+HFN*MAC_HF_Length)*TS_Length

The SACT time units depend on the implementation and are given by theparameters MAC_FW Accuracy or MAC_SACT_Resolution. The span of thisrelative clock is given by Span of MACclock=MAC_HFC_Max*MAC_HF_Length*MAC_TS_Length

At each overflow of the Hyperframe Counter, the Relative ITP timestampneeds to be updated as follows:(New value of RITP)=(Old value of RITP)+(Span of MAC clock)

An operation of generating an absolute timestamp is needed when the MAClayer has to inform the LLC layer that a new real time clock value hasbeen delivered by the network. The timestamp value computed in suchinstance is the absolute time at the moment the MAC layer sends to theupper layers the indication of new timestamp arrival, as follows:Absolute ITP timestamp=(TSN+HFN*MAC_HF_Length)*MAC_TS_Length+RITP

Relative to generation of a value for the Relative ITP timestamp, whenthe application communicates an ITP timestamp value to the MAC layer,the MAC layer will need to reference this timestamp to the MAC clock(TSN and HFN) and store the resulting value in its Relative ITPtimestamp register. This will occur for instance in a Cell Relay whenthe application will need to true up the real time clocks of the cell.The RITP value will be computed with the following equation:RITP=(ITP timestamp value)−(TSN+HFN*MAC_HF_Length)*MAC_TS_Length

Relative to present protocol subject matter Time SynchronizationServices, there are two ways to propagate the time over an entire cell:at synchronization phase and by periodical update. The present absolutetime subject matter will be used both inside the present networkprotocol subject matter itself (at the MAC layer) and at the applicationlevel (in this example, energy metering).

Each cell relay has an NTP client which allows it to receive an NTPtimestamp from the WAN. It uses this NTP value to update its RITP. Thecell relay sends periodically ITP broadcast messages to the entire cell,with exactly the same process as a “regular” broadcast message. This ITPbroadcast message contains the RITP information, base of the timere-generation in endpoints. Each time an EP receives such a message, itreads and updates the RITP and forward the message to its sons.

The second way to acquire the RITP field is during the synchronizationprocess. When an EP wakes up after a power failure, it no longer has anynotion of time. The RITP field is given inside a SYNC ACK message afterthe EP requests synchronization with a synchronized neighbor. Thus, assoon an EP is synchronized, it advantageously knows the time per thepresent protocol subject matter.

In one aspect of the present subject matter, an objective advantageouslyachieved is to true up the real time clocks in every node of a cell.There is no requisite Input argument. The operation is described in thecontext of a service that is only used in a Cell Relay. The Cell RelayMAC layer builds an ITP broadcast packet. The MAC header of this packetcontains the value of the RITP together with the HFN at the moment thepacket is created. This packet is broadcast with the usual broadcastrules defined in this protocol. This broadcast will allow each recipientnode to update its own Relative ITP timestamp. The recipient nodes willuse the HFN value included in the packet to detect a possible overflowof the MAC clock since the ITP broadcast packet creation. The span ofthe MAC clock should be much longer than the expected travel time of apacket through the meshed network in order to avoid ambiguities.

In another aspect of the present subject matter, an objectiveadvantageously achieved is the provision of a service which allows theapplication layer (through LLC and NET) to update the Relative ITPtimestamp of the MAC layer. The requisite Input argument involves theAbsolute ITP timestamp. The operation is again described in the contextof a service that is only used in a Cell Relay. The MAC layer uses theabsolute ITP timestamp to compute a Relative ITP timestamp value. TheMAC layer then updates its RITP register with this computed value (See“Generation of a value for the Relative ITP timestamp” above). Thisservice is usually called before an ITP broadcast. It is distinct fromthe ITP broadcast service in order to avoid the uncontrolled aging of atimestamp in a packet waiting in a queue.

Still another aspect of the present subject matter is advantageousachievement of an objection of indicating to the application layer(through LLC and NET) that an ITP broadcast has been received. Therequisite Output arguments are Absolute ITP timestamp, RITP, and HFN.The description of the operation is that the recipient endpointretrieves the RITP and HFN values from the MAC header. It then comparesthe HFN value in the message header to its own HFN. This allows the MAClayer to detect a possible overflow of the Hyperframe Counter since thecreation of the ITP broadcast packet. If no HFC overflow occurred, itwrites the RITP value in its own RITP register. If an overflow occurred,it increments the RITP value with the span of the MAC clock and writesthe result in its RITP register. The MAC layer then computes an absoluteITP timestamp (See “Generation of an absolute timestamp” above) andsends to the LLC layer an indication with this absolute ITP timestamp asargument. This indication informs the LLC layer that the RITP has beenupdated in the MAC layer and gives to the LLC layer the value of atimestamp that can be used to update the real time clock of theapplication. The ITP broadcast packet is then forwarded to theendpoint's sons according to the usual broadcast rules. The RITP and HFNvalues retrieved from the ITP broadcast packet MAC header are also sentto the LLC layer with the purpose of allowing the reconstruction of thepacket for the follow-up of the broadcast.

Another objective advantageously achieved with the present protocolsubject matter allows an endpoint to obtain the value of the RITP andthe value of the Hyperframe counter from its father at synchronization.Such operation is actually a part of the synchronization process, asotherwise discussed herein. However, for the sake of present clarity, itis simply noted that when an endpoint receives an acknowledgement to itssynchronization request, it will update its RITP and HFN from theinformation contained in that acknowledgement.

An important aspect in a mesh network using a frequency hopping sequenceis the synchronization process. Indeed, once every EP in the cell knowsthe channel sequence and the current TS in the sequence, they need toperiodically maintain such information up-to-date. Due to clock drift,such information can become corrupted with time. Re-synchronization ofthe clock of every EP is therefore needed.

When the subject network is switched on for the first time, nocomponents know when the time slots begin and which frequency to use. Asin all synchronized systems, a metronome or equivalent operation isneeded. The cell relay (or Cell Master) is the preferred such componentin the present protocol subject matter because it is always consideredas “synchronized”. For the other endpoints, per the present subjectmatter, the synchronization is hierarchical. Endpoints which can hearthe relay (Cell Master) become synchronized, and then it is their turnto synchronize their neighbors. During such process, a level is given toeach endpoint, which indicates how many hops they are from the cellrelay (Cell Master).

A relay has a level “1”; a non-synchronized endpoint has a level “0”;and an endpoint that is at N hops from the cell relay has a level “N+1”.The respective levels relative to present synchronization protocol arerepresented in present FIG. 18.

To summarize terminology as otherwise referenced herein, an endpointwhich is:

-   -   Level N compared to an EP level N−x, x>=1, is called a son (or        child)    -   Level N compared to an EP level N+x, x>=1, is called a father        (or parent)    -   Level N compared to an EP level N is called a brother (or        sibling)    -   Level 0 is called an orphan

Present FIG. 19 represents the hierarchical synchronization aspects ofthe present subject matter, such that an endpoint keeps itssynchronization from any synchronized neighbor that respects thefollowing conditions:

-   -   The neighbor should belong to the same cell (same channel        sequence)    -   The neighbor should be a father, i.e. it should have a higher        hierarchical position (a lower level number).        Such conditions give the possibility for an endpoint to have        more than one source for synchronization information. This is        possible because, at the end, everybody in the cell will have        the same time base. It also allows a level N endpoint to        synchronize itself with a level N−2 endpoint (see the conditions        to change level as otherwise referenced herein).

The maximum level in a cell is 63. It is limited by the number of bits(6) allocated to this field in the messages. As a result of the rulesabove, an EP of level 63 cannot be used for synchronization.

Present FIG. 20 represent various aspects of the present protocolsubject matter as relates to resynchronization between endpoints (EPs).Per present subject matter, an EP advantageously resynchronizes itselfeach time it receives a message from one of its fathers, by re-computingthe beginning of its next time slot. At each beginning of time slot, acountdown timer called Slotted Aloha Countdown Timer, SACT, is set withthe MAC_TS_Length value. When such timer reaches 0, the MAC layerswitches to the next timeslot. The resynchronization process consists inre-computing the value of the SACT to align the sons' timeslots on thefathers' ones. This resynchronization is performed with the followingalgorithm:

-   -   The length of the preamble (including the SFD field) is        predefined and the bit rate is known. Therefore, the duration,        d1, of the preamble can be readily computed.    -   On the transmitter side, the sender indicates in the MAC header        the SACT value, SACT10, corresponding to the beginning of the        physical transmission of the packet (i.e. the time between the        transmission of the first bit of the packet and the next time        slot change, as measured in SACT time units).    -   On the receiver side, as soon as the SFD is detected, the        physical layer of the recipient endpoint reads its timer value,        SACT20, and deduces SACT20+d1, the SACT value when the sender        began the transmission of its message.    -   The physical layer indicates to the MAC layer that it has        received a message at SACT20+d1. The MAC layer also reads the        timer value SACT10 contained in the MAC header.    -   Then at any moment, the MAC layer can update its timer value,        SACT2, by added the correction ΔSACT.        SACT′2=SACT′2+ΔSACT        ΔSACT=SACT10−(SACT20+d1)

Advantageously per such present operations, the endpoint auto adjuststhe next time slot, which compensates for the internal drift of thedevice. By following such present process at each received message froma higher-level endpoint, the endpoint drastically decreases theprobability of losing its synchronization.

Each time an endpoint receives a message from a neighbor endpoint, theMAC layer records two time values in the neighbor table: the SACT valueread from the sender MAC header (SACT10 above) and the reception time ofthe packet. These values can then be used at any moment when theendpoint decides to synchronize with that neighbor.

The SACT is defined with a resolution of MAC_SACT_Resolution μs.

Dating received and transmitted messages should be accurate enough toallow the present protocol to work properly, and especially for theresynchronization process to work correctly. The crystal clock of thesystem should be chosen at ±MAC_Crystal_Accuracy ppm and the firmwarehas to date the message at ±MAC_FW_Accuracy μs. Dating a receivedmessage should be done by taking a snapshot of the SACT countdown timerwhen the SFD field is detected by the PHY layer, as it is otherwiseexplained herein.

A sender should also date the message and include the SACT value in theMAC header. It is a difficult task to precisely compute the SACT at theexact moment when the PHY layer will send the first bit. Indeed, in themeantime, the MAC layer will have to build the header, compute the CRCand then give the message to the PHY layer; and then the PHY layer willhave to add its header and configure the RF transmitter. It is an aspectof the present subject matter that it is preferable for a givenimplementation thereof (such as in producing the actual firmware to beused), to estimate a worst case time between the moment of the datingand the planned transmission, and set a timer with such time. Duringsuch allocation of time, the MAC and PHY layers should have enough timeto prepare the packet. The PHY layer will then wait during the remainingtime and send the first bit as soon as the timer reaches the definedvalue.

For a particular implementation using an off-the-shelf transceiverapproach, it is noted that dating at the SFD reception is typically themost simple and preferred thing to do. If it is more convenient for agiven implementation to date at another moment inside the message, theuser is free to do so per the present subject matter, as long asremembering to take into account the length of the message.

When an endpoint receives a message, it can easily compute the beginningof the next time slot. But such information alone is not enough sincethe channel of the next time slot is not known at that moment. Theendpoint will find this information in the MAC header. Three fields areimportant for the synchronization process. The first one is the level ofthe sender: (Re)-synchronization is only allowed with the fathers. Thetwo other fields are the time slot number and cell address. As everyendpoint can compute the channel sequence from the cell addresscontained in the header, and because the time slot number informs onwhere the sender is in this hyper sequence, the recipient can find thechannel that the sender will use in the next time slot.

Such three fields are present in all the messages: beacons or othermessages. So every message can be used for synchronization. The timeslot number field has another use. Even if an endpoint receives amessage on an adjacent channel, it will know the actual channel on whichthe message has been sent.

An EP re-synchronizes itself each time it receives a message from a SYNCfather, whatever the nature of the message. If there is no traffic,dedicated messages (presently referred to as beacons) are sentperiodically by every synchronized EP. If the traffic is dense enoughcompared to the default beacon periodicity, beacons are not sent. It canbe viewed as a timer with the initial value MAC_Beacon_Periodicity_SYNC.Each time a packet is sent, the timer is restarted from the beginning.If the timer reaches ‘0’, a beacon is sent.

Several parameters permit the computation of beacon periodicity ofsynchronized EPs. The most important is the clock accuracy of the EP. Itis mainly dependent on the accuracy of the crystal (or oscillator) andof the firmware clock. Another parameter is the number of beacons onecan assume a system can miss, due to collisions, jammers, etc. The lastparameter is the maximum drift the system is authorized to have between2 levels. For such computation, one may consider the worst case, when anEP has only one SYNC father.

${{Beacon\_ Periodicity}{\_ SYNC}} = \frac{TS\_ Margin}{\begin{matrix}\begin{matrix}{2*{Clock\_ Accuracy}*} \\{\left( {{{Max\_ Nb}{\_ of}{\_ Missed}{\_ Beacons}} + 1} \right)*}\end{matrix} \\{TS\_ Length}\end{matrix}}$

EXAMPLE

-   TS_Margin=15 ms-   Clock_Accuracy=±20 ppm-   Max_Nb_of_Missed_Beacons=3 missed beacons-   TS_Length=150 ms-   −>Beacon_Periodicity_SYNC=625 TS (this corresponds to 1 min 34 s)

In a low traffic situation with many endpoints transmitting periodicbeacons, there is a significant probability that two endpoints willchoose the same time slot and sub time slot to transmit their beacons.This probability increases roughly as the square of the number ofendpoints and would be close to one in a cluster of 100 endpoints. Thiswould lead to recurring collisions between those beacons. To preventthis situation, the actual transmission time of the beacons should berandomized within a ±20%, i.e. around 125 time slot (forMAC_Beacon_Period_SYNC=625) window while keeping the average periodicitydefined above.

The Cell Size Indicator, CSI, is a 4-bit field contained in each MAClayer. This field value is set by the Cell Master depending on the sizeof the cell (determined by the number of entries in a NET Cell Masterrouting table). This will require an internal message from the NET layerof the Cell Master to its MAC layer. This field, like the GPD, willpropagate with each message from the Cell Master. At each receivedmessage from one of its fathers, or from endpoints which were fathers,the node updates its CSI looking at the MAC header value. An endpoint(other than the Cell Master) always keeps the highest value of CSI amongits fathers belonging to the same cell.

The algorithms to choose a cell during discovery phase and to switchcell will consist in selecting the best father according to their term.The CSI is one of the parameters used to determine this term. The valuesfor the CSI are given in the network layer section.

Alternatively per the present subject matter, the CSI propagation couldbe based on the last message received from a father. Such approachavoids keeping this extra field in the neighbor table of each EP. Thetradeoff is that during the propagation of a new value of CSI, therewill be much more bounces (the other way makes it faster to increase thevalue and slower to decrease it).

A neighbor of an endpoint is called a potential synchronization father(or potential SYNC father) for that endpoint if it complies with all ofthe following conditions:

-   -   The neighbor is known to the endpoint, i.e. it has been heard at        least once and is still recorded in the neighbor table.    -   The neighbor is already synchronized (its level is different        from zero). The neighbor has a level below 63 (which is the        maximum level allowed in a cell).    -   The neighbor is registered to a cell (RS bit=1).    -   The neighbor is an authorized endpoint, i.e. an endpoint that        has not been flagged as forbidden (The Sync_Forbidden bit in the        neighbor table should be zero).    -   The neighbor has at least a weak connectivity. Connectivity        Degree≧1 (CD value≧1)    -   The neighbor belongs to a cell which is not forbidden.    -   The neighbor belongs to a cell which is not full (CSI value        different from the maximum value). Note: This condition is not        considered if the node is already a member of this full cell.

The forbidden character associated with a cell is set by the API layer.It can be set if the user decides that two close-by cells should neverauthorize endpoints from the other cell or if a cell is full. A cell canbe re-authorized by the API layer. This information will also be resetif the meter stays non-synchronized for a long time, defined by theparameter MAC_Unsynchronized_Timeout.

Through a selection process described later, the more suitable potentialsynchronization father is chosen to become the synchronization father(or SYNC father for short). If this neighbor answers positively to thesynchronization request, it becomes the actual SYNC father of the node.

It should be pointed out that the potential SYNC father conditions areevaluated at a given time. If a potential SYNC father (or a SYNC father)is not heard during MAC_Neighbor_Timeout, it will be removed from theMAC neighbor table and will be no more considered as a potential SYNCfather (or SYNC father).

When an endpoint becomes synchronized with a cell, some of its potentialfathers will be actual fathers and some others might be brothers orsons. On the other hand some fathers might not qualify to be potentialSYNC fathers. The fathers that are also potential SYNC fathers will becalled healthy fathers. Of course by definition a non synchronizedendpoint has no father at all, it can only have potential SYNC fathers.FIG. 21A schematically represents Potential SYNC fathers and healthyfathers for a synchronized node.

The Connectivity Degree (CD) is a variable that measures theconnectivity of a node with the network. Connectively Degree per presentsubject matter is represented by present FIG. 21B. The CD value of anode depends only on the number of potential SYNC fathers it has amongits neighbors. If the endpoint is not yet synchronized, all potentialSYNC fathers are considered for the computation of CD. On the other handif the endpoint is synchronized, only the brothers and fathers are takeninto account. Notice that a synchronized endpoint should have at leastone father (inferior level) in order to be considered havingconnectivity (CD>0). The table below shows how a value is assigned to CDas a function of the number of potential SYNC fathers. This degree isindicated in most of the MAC headers and shared with the neighborhood.It will be used by others for synchronization decisions.

In order to maintain its clock synchronized with the network, anendpoint must receive frequent enough beacons or messages from itsfathers. There is therefore a need to assess the average rate at whichan endpoint receives messages from a given neighbor. This will play animportant role when deciding which neighbor can act as an efficient SYNCfather. Neighbors that are only heard once in a while will be deemed badpotential SYNC fathers compared to others that are regularly heard. TheReception Rate Indicator (for short RXI) is easy to implement andprovides an indication of the rate at which messages are heard from aneighbor, although it is not an exact measurement of the actualreception rate.

A variable of one byte is associated with each neighbor in the neighbortable. We call this variable the RXI of the neighbor X and write RXI(X).These RXI are managed according to the following rules:

-   -   Upon reception of a message from X, increment its RXI        -   RXI(X)←Min[RXI(X)+RXI_Increment, 255]    -   At each hyperframe start, decrement all the RXI        -   RXI(X)←RXI(X)*RXI Decay, for all Xε{Neighbor table}

High values of RXI indicate that the frequency of message reception ishigh. Therefore, neighbors with high RXI values have an advantage in theSYNC father selection process.

Other discussion herein reflects on how an EP keeps its synchronizationfrom the cell clock per the present subject matter, which assumed forsuch point of discussion that the EP was already connected. At power-upof after a loss of synchronization, an EP is considered asnon-synchronized and enters in a so-called Discovery Phase.

In other words, per present subject matter, this aspect has to do withthe provision of a network discovery arrangement wherein a new node withno previous knowledge of its environment is able to establish a link toan existing network. Upon wake up, such new node will preferablytransmit a discovery beacon over several channels in succession and thengo into a listening mode to listen for any response. The transmitteddiscovery beacon includes information as to a specific channel on whichthe new node will listen. When synchronized nodes hear the discoverybeacon, they transmit a message to the new node including informationnecessary for the new node to synchronize with the network. Thetransmitted message may include time, frequency, network identifier,etc. and is transmitted at the new node indicated frequency and atrandom times within a listening window. The new node may then collectinformation and choose the best network among desired criteria andsynchronize with the chosen access node in the network.

In a frequency hopping network, nodes that just powered on and startfrom scratch have no idea of their environment. They need to connect andto synchronize with a network, which is complicated by the fact theydon't know at which frequency and which time the network operates. TheDiscovery Phase of the present protocol subject matter is an algorithmicapproach to enable the node to quickly analyze its environment and lookfor the best network it can connect with, without disturbing theoperating network.

Broadly speaking, additional benefits of such present subject matterinclude that a new neighbor finds a connection with a network in a veryshort time, all the neighborhood networks are discovered, and all whilethe neighborhood operating networks are not disturbed from their regularactivities.

More specifically, as the traffic is very low inside a cell and as thecell is continuously switching from one channel to another, it can takea long time for a non-synchronized endpoint to intercept a message froma synchronized one. To accelerate this process, the non-synchronizedendpoint sends discovery beacons successively on all the channels. Theorder of channels follows the hopping sequence generated by a cell ID of0. There is one discovery beacon per channel in the system. The channelof the first discovery beacon should be computed randomly to ensure thattwo non-synchronized endpoint will not choose the same at power up.Discovery beacons are sent every MAC_Discovery_Beacon_Period ms.

Each discovery beacon contains in the MAC header the Number of remainingDiscovery Beacons (NDB field) to send, and the listening channel (RxCfield). After sending all the discovery beacons, the endpoint enters ina listening state during MAC_Listening_Window_Length. The listeningchannel is the same than the one used for the first discovery beacon.There is a high probability that synchronized EPs in the radio range ofthe EP will receive at least one of these discovery beacons. Thereception of one of these discovery beacons forces them to send a“forced beacon” at the required channel inside the listening window. Asevery synchronized EP in the neighborhood will do the same pattern, thelistening window should be long enough to contain most of the answers.The formula below gives the length of the listening window, for a casewhere the number of collisions between the neighbors' answers isminimized.

${{Listening\_ Window}{\_ Length}\mspace{14mu}({seconds})} = {\left( \frac{{Max\_ Nb}{\_ of}{\_ Neighbors}}{{Nb\_ of}{\_ Sub}{\_ TS}} \right)*{TS\_ Length}}$

EXAMPLE

-   MAC_Max_Nb_of Neighbors=100 EPs-   MAC_Nb_of_Sub_TS=6 Sub_TS-   MAC_TS_Length=150 ms-   −>MAC_Listening_Window_Length=2.5 seconds or 17 TS

Present FIG. 22 represents a Discovering phase example with basicfrequency hopping sequence number 0 for an embodiment operative inPHY-FHSS-NA-915.

During the listening state, information on all the neighbors that haveanswered, and mainly synchronization information (address, level, time,channel, cell address, GPD, and RSSI), is saved in the neighbor table ofthe endpoint. At the end of the listening state, if there is no answer,the next discovery phase is started after a random time (see below). Thechannel of the first beacon of this new discovery phase is the next one,in the hopping sequence, after the one used in the previous listeningwindow. This process is repeated with a period ofMAC_Min_Discovery_Phase_Period modulated by a random time (maximumdefault value is 100 ms) to avoid repetitive collisions betweennon-synchronized endpoints. Between the end of the listening window andthe start of the new discovery phase, the endpoint can stay in listeningmode. This process stops at the end of a listening period if theendpoint has received at least one answer from a potential SYNC father(if it is a warm start then this answer must come from its preferredcell; see present discussion about warm vs. cold start).

In the situation that an endpoint doesn't succeed to synchronize withone cell after MAC_Max_Nb_of_Discovery_Phases discovery phases, theperiod of discovery phases will be increased fromMAC_Min_Discovery_Phase_Period to MAC_Max_Discovery_Phase_Period. Thiswill prevent an orphan from polluting the RF band with useless messages.The period will be reset to its initial value if the endpoint succeedsto synchronize.

At the end of the listening window, if at least one valid answer hasbeen received, the EP goes into the next step. An answer or data messagefrom a neighbor will be considered as valid for synchronization purposeif it meets the potential SYNC father conditions. It can be pointed outthat non-synchronized endpoints, non-registered endpoints (RS=0),endpoints of level 63, non-connected endpoints (CD=0) or endpoints froma full cell (CSI=max value) are not allowed to send forced beacons. Butthere is a chance that an endpoint in discovery phase hears a messageintended for another endpoint, in which case it should check thepotential SYNC father conditions.

The next step for this endpoint that tries to become synchronized is tochoose the best access to the network. For this selection the endpointwill consider all neighbors it has received an answer from or overhearda packet from, unless they were discarded for the aforementionedreasons. Among these neighbors, it will choose the best access using acriterion based on the following principles:

-   -   Low CSI: Cells with a lower number of endpoints will be        preferred to the ones that are more populated.    -   Low EP_GPD: Neighbors with low GPD values will be preferred. The        EP_GPD of a neighbor is the Global average Propagation Delay        between the endpoint and the Cell Master through the selected        neighbor.    -   Good Level: a neighbor closer to the Cell Master will be        preferred to a neighbor more hops away from the Cell Master.

The level of a neighbor, as well as its CSI, is indicated in the MACheader. The EP_GPD values, on the other hand, need to be computed.Mathematically, EP_GPD=GPD of the neighbor (as reported in its MACheader)+LPD+MAC_GPD_TD.

The Local Propagation Delay (LPD) of a neighbor is usually computed fromthe track record of past communication attempts with that neighbor. Thisalgorithm is explained in a dedicated chapter. During the discoveryphase, computing a LPD is however impossible because the endpoint hasnot yet exchanged enough messages with the neighbor to gatherstatistically relevant information. In this case an estimation of theLPD based on the RSSI reading is used.

In order to make a selection based on a combination of the principlesmentioned above, we introduce a term to characterize the ability of aneighbor to act as a suitable synchronization source.SYNC_Disc_Merit=EP_GPD*MAC_SYNC_Disc_Weight_GPD+MAC_SYNC_Disc_Weight_Level*LVL+MAC_SYNC_Disc_Weight_CSI*CSI

There are defined three MAC layer parameters, MAC_SYNC_Disc_Weight_GPD,MAC_SYNC_Disc_Weight_Level and MAC_SYNC_Disc_Weight_CSI. The values ofthese parameters will depend on the importance one wants to give to theGPD, level or to the cell size in the selection process. If one sets thelast two parameters to zero, only the GPD will be used to select thesynchronization point.

The selection process for the best access can be summarized as follows:

-   -   1. First the endpoint computes the EP_GPD for each neighbor it        has received an answer from or overheard a packet from (provided        the neighbor is a potential SYNC father).    -   2. For each neighbor, the endpoint computes the term for        synchronization, SYNC_Disc_Merit.    -   3. The endpoint selects the neighbor with the lowest        SYNC_Disc_Merit value and synchronizes its time slots and        frequency sequence with it. The neighbor table must be managed        carefully in this process to allow the endpoint to back up and        re-synchronize with another neighbor if necessary.    -   4. The endpoint then has to ask this neighbor for the permission        to synchronize with it. For this purpose it sends a specific        message called a synchronization request (SYNC Request).    -   5. If this request is not positively acknowledged, the endpoint        should flag the neighbor as forbidden (set the Sync_Forbidden        bit to 1 in the neighbor table) and proceed to step 3 above with        the next valid neighbor in the list with the best term. Several        situations can lead to this failure:        -   a. The neighbor doesn't answer the SYNC request. The            endpoint's MAC layer will repeat the request (for a number            of attempts defined by Max_Nb_of SYNC_Request and with a            minimum period given by MAC_SYNC_Request_Period plus some            randomization over a few time slots) before concluding that            the neighbor is not reachable.        -   b. The neighbor answers with a SYNC NACK to indicate that it            refuses the synchronization request. In this case the            endpoint must immediately conclude that this neighbor is not            suitable for synchronization.    -   6. If the neighbor accepts (by sending a SYNC ACK), the endpoint        updates its level and switches to synchronized mode. The SYNC        ACK also contains the current hyperframe number in the cell        (HFN) and the Relative ITP (RITP). The RITP will allow the        endpoint to know the absolute time without waiting for an ITP        broadcast message (see Time section).

The table of present FIG. 23 gives an example for the use of the termwith three neighbors and MAC_SYNC_Disc_Weight_GPD=1,MAC_SYNC_Disc_Weight_Level=50. In this example the preferred neighbor isthe third one.

The foregoing discussion described the mechanism for a cold start, i.e.the non-synchronized endpoint had no prior knowledge of the network.When an endpoint which is already synchronized and registered with acell experiences a power outage and is then powered on again, it can useits knowledge of the network to recover more quickly its state frombefore the power outage. This process is called a warm start.

For a warm start, there will be a notion of preferred cell at the MAClayer level. The preferred cell is the one the endpoint was registeredwith before the power outage. At first, the endpoint will consideritself already registered (RS bit set to 1) and try to connect only toits previous cell. If after a number of discovery phases (defined byWarm_Start_Discovery_Duration) it has not managed to do so, it willconsider the other cells and restart the discovery phase withoutpreferred cell as in cold start.

During the warm start, the discovery beacons contain the address of thecell that the endpoint wants to synchronize with. This is to preventendpoints belonging to other cells from sending forced beacons andflooding the link at each discovery phase for nothing. This field is setto 0 during cold start.

It is very important that the selected neighbor checks its ownconnectivity to the network before acknowledging a SYNC request. Beforeacknowledging a SYNC Request an endpoint must check if it has received arecent message from a healthy father during the lastMAC_SYNC_Father_Request_Beacon_Threshold time slots. If it is not thecase, it should request a beacon from the healthy father with the bestLPD:MAC_SYNC_Father_Request_Beacon_Threshold≦Beacon_Periodicity_SYNC

Upon reception of a SYNC Request an endpoint will either answer with aSYNC ACK (or SYNC NACK) or send a beacon request to one of its healthyfathers if it has not received any recent message from any of them. Inthis last case the endpoint doesn't answer the synchronization requestimmediately but waits for the neighbor to retransmit its request. By thetime the same neighbor requests again synchronization, the endpointshould be able to accept the request because it will have received arecent messages from one of its own healthy fathers. In this case theendpoint will send a SYNC ACK.

The endpoint which has received the synchronization request needs eitherto send the synchronization acknowledgment or to request a beacon in thetime slot following the SYNC Request reception.

The father which has been requested to send a beacon, has to send it inone of the next time slots following the reception of the beaconrequest, before MAC_SYNC_Request_Period time slots have elapsed. If thisnode has already planned to send another message (that has the sameheader information as a beacon) in this window, it doesn't need to senda beacon.

The endpoint which has been asked for synchronization will send a SYNCNACK in the following cases:

-   -   The endpoint is not synchronized anymore (level 0)    -   The endpoint has a level 63.    -   The endpoint is not registered to a cell (RS bit=0)    -   The endpoint has no connectivity. Connectivity Degree=0 (CD=0).        The Connectivity Degree should be updated after the SYNC Request        reception, mainly to refuse synchronization in the case where an        endpoint tries to synchronize with its own (former) son.

Upon reception of a SYNC NACK from a neighbor, its Sync_Forbidden bitshould be set to 1, which prevents further synchronization requests frombeing sent to this neighbor. Between successive discovery phases, theneighbor table should not be cleared in order to retain thisinformation.

The forbidden bit associated with a neighbor should be cleared to 0 ifthe node notices a change in these parameters:

-   -   The neighbor changes its level (except to 0).    -   The neighbor switches to another cell.    -   The neighbor becomes registered (RS bit from 0 to 1).    -   The neighbor changes its connectivity degree (CD from 0 to a        positive value)    -   The neighbor is new in the neighbor table.

The forbidden bit of all the neighbors in the table should be cleared to0 if:

-   -   The node changes its level (except to 0).    -   The node switches to another cell.

A complete synchronization example is provided with reference to presentFIG. 24. EP6 is a new meter and it has three neighbors on two differentcells. EP4 is the best endpoint to synchronize with. In this example,there are only seven different channels.

If the only SYNC father requests synchronization from one of its son,the son has to refuse immediately. The son should also realize that ithas lost its synchronization. A meter, which has refused asynchronization, has to be marked (Sync_Forbidden=1) not to be askedlater again. If the properties of this meter change (level, cell, powerup), the mark is removed.

The present protocol subject matter advantageously provides for BeaconRequests and RS bit resolving to avoid circular routes. Such subjectmatter primarily applies to the environment of a tree network where thenodes are organized in cells with a concentrator (or root node) at the“head” of each cell. Each node has a level associated with its place inthe cell. As otherwise referenced herein, the root node is level 1 andis always synchronized (by definition). In order to get its data to theroot node a node must be synchronized in the corresponding cell.

Stated another way, the present synchronization process requires a handcheck between one synchronized node and another node. A node whichsynchronizes on another becomes its son and the other node becomes thefather of the first one. The new node level is one more than the one ofits father. Therefore, all nodes have a level above 1. Nodes of the samelevel are called brothers. The group of fathers, brothers and sons of anode are called its neighbors. Each node keeps a table of its neighbors.

The problem successfully addressed by the present subject matter relatesto when a node (node 1) looses its synchronization and asks one of itsbrothers or sons for synchronization. If one of these nodes (forexample, node 2) accepts to give synchronization to node 1, then itchanges level (level of node2+1). After that, another node (for example,node 3) which had node 1 as father can realize it is no longer the case(since level of node 1 is now over its own level) and can try to find anew father to which to synchronize. Specifically, it can ask node 1,which will accept. If node 3 was the father of node 2, it will startfinding a new father. Left on its own, such process can become anendless loop with nodes asking synchronization to one another without areal path to the concentrator (and thus without being actuallysynchronized). The main part of this problem is the delay between thenew state of a node and the time its neighbors realize it.

The solution of the present subject matter is based on Synchronizationinformation, which is present in all messages; Beacons (which arepackets with only the synchronization information); and based on beaconrequests (which are packets requesting a beacon from a neighbor).

One of the main parts of information for synchronization takes the formof one bit (the RS bit) which indicates if a node still has fathers(that is, the RS bit set to 1) or not. This bit is present in allpackets because this information must be updated as fast as possible andthus must make use of any communication. A node will accept to givesynchronization only if it has received a relatively recent message froma father (with an RS bit set to 1).

When a node receives a synchronization request (SYNC_REQUEST), it willcheck if it has received a recent message from one of its fathers (withRS bit set to 1). If it finds such a father then it accepts to givesynchronization (SYNC_ACK). Otherwise, it sends a Beacon Request to oneof its fathers with the RS bit set to one. This father will send abeacon with all its synchronization information (including RS bit) inresponse. The node asking for synchronization will repeat its demand andthis time the node receiving the request should have received the beaconand should be able to send a synchronization acknowledgement (SYNC_ACK).If a node has no father with RS bit set to 1 it will refusesynchronization (SYNC_NACK).

The beacon request allows nodes to refresh the information of theirneighbors when they consider it is not recent enough, especially in thecase another node asked them synchronization (they must be sure to stillhave a good connectivity before accepting). Such present solutionadvantageously provides a relatively fast way to propagate thesynchronization information between nodes, thereby avoiding that theycreate a virtual circular net without an actual root. The beacon requesthelps refresh the knowledge of a node on its neighbors if theinformation is considered too old.

Present FIG. 24 illustrates a configuration example while present FIG.25 represents a Synchronization process, both relative to a completesynchronization example per the present protocol subject matter. Withfurther reference to present FIGS. 24 and 25, EP6 is a new meter and ithas 3 neighbors on two different cells. EP4 is the best EP tosynchronize with (best level, best GPD). In this example, there are only7 different channels.

EP6, at power up, is level 0, not synchronized, and enters into itsdiscovery phase. It sends successively beacons on the 7 channels, andits 3 neighbors receive each one a beacon, because time and frequencymatch at one lucky moment. After sending all the beacons, EP 6 enters ina listening state at a frequency that it has indicated in the formerbeacons. The 3 neighbors react to this stimulus by sending a “forced”beacon a few (random) time slots later at the required frequency. EP 6,which listens on the green channel receives these beacons and savessynchronization information. It should be noted that during thislistening phase, EP3 can intercept messages from other EPs. Due tooperation of the MAC header, EP3 would be able to save theirsynchronization information as it does with its 3 current neighbors.

At the end of this listening phase, it is time for synchronization.Accordingly, EP6 adjusts its time slots on EP4, and requestssynchronization. EP4 checks that it still has a connection with the cellrelay 1, its Sync father, by requesting a beacon, and as soon itreceives the beacon, sends a SYNC ACK to EP6. EP6 becomes synchronizedand becomes level 3 in the cell number 1.

Note that EPs numbers 3, 4, and 5 have broken their channel sequencesduring 1 TS to send a forced beacon on the green channel. It is not anissue because if another EP has heard them at this moment it would haveread, in the header, the channel that would have be used (CELL address,and TS number). The fact that it is another channel that has been chosenis transparent for the MAC Layer.

Present FIG. 26A represents one example of an Initial Configuration, andpresent FIG. 26B represents an example of a new endpoint, bothillustrative of circumstances of one endpoint finding a better endpointfor communication purposes, per present subject matter as furtherdiscussed herein.

Each endpoint should indicate in the MAC neighbor table which one of itsfathers has sent the SYNC ACK to grant synchronization rights. The SYNCFather flag serves this purpose.

It may happen that the communication of a node with its SYNC father orthe characteristics of the SYNC father deteriorates to the point that anew SYNC father needs to be found. Two cases need to be considered.

-   -   1. The communication with the SYNC father deteriorates but the        SYNC father is still a healthy father because it complies with        the potential SYNC father conditions. In this case, when the        endpoint runs its periodical SYNC father selection process, it        may chose to discard the old SYNC father and pick up a new one        which will be a better access to the network.    -   2. The SYNC father disappears or loses its SYNC father status.        We call this a SYNC father loss, it will happen if:        -   The SYNC father remains silent for too long and disappears            from the neighbor table (MAC_Neighbor_Timeout).        -   The SYNC father doesn't comply anymore with the potential            SYNC father conditions.        -   The SYNC father changes its level.        -   The SYNC father moves into another cell.

In this case a SYNC father selection process is immediately triggered.It can be noted that the selection process can lead to the same fatheras before, the selection being made according to the potential SYNCfather criteria and the subject term.

If a node changes its level after the selection of a new SYNC father,then all flags should be removed except the last one set (for the fatherthat just sent the SYNC ACK and allowed the endpoint to change itslevel). An endpoint should have only one father with the SYNC Fatherflag set. In this case the endpoint is considered synchronized. Notethat a father that is not good for synchronization can still be used forrouting messages (if it is still a father).

In order to compare the relative worth of neighboring endpoints assynchronization fathers, we consider a term regarding SYNC_Merit, whichis defined bySYNC_Merit=EP_GPD+SYNC_Penalty_LPD+SYNC_Penalty_RXI+SYNC_Penalty_CD

This term is only computed for the neighbors that are potential SYNCfathers (see potential SYNC father conditions). The main component ofthis term is the EP_GPD. Additional terms are introduced to decrease theprobability of choosing some neighbors that may be less suitable assynchronization fathers. The SYNC_Penalty_LPD term is necessary becausethe LPD has a finite range. When the LPD of a neighbor is truncated toits maximum value, LPD_Max, the actual propagation delay is not knownand a constant is added to the term to take into account the risk ofselecting that neighbor as a SYNC father. The SYNC_Penalty_LPD term isdefined by:

${{SYNC\_ Penalty}{\_ LPD}} = \left\{ \begin{matrix}0 & {{{if}\mspace{14mu}{LPD}} < {LPD\_ Max}} \\{SP\_ LPD1} & {{{if}\mspace{14mu}{LPD}} = {{{LPD\_ Max}\mspace{14mu}{AND}\mspace{14mu}{RSSI}} > {{- 80}\mspace{14mu}{dBm}}}} \\{SP\_ LPD2} & {{{if}\mspace{14mu}{LPD}} = {{{{LPD\_ Max}\mspace{14mu}{AND}}\mspace{14mu} - {100\mspace{14mu}{dBm}}} \leq {RSSI} \leq {{- 80}\mspace{14mu}{dBm}}}} \\{SP\_ LPD3} & {{{if}\mspace{14mu}{LPD}} = {{{LPD\_ Max}\mspace{14mu}{AND}\mspace{14mu}{RSSI}} < {{- 100}\mspace{14mu}{dBm}}}}\end{matrix} \right.$

The SYNC_Penalty_RXI term is necessary to avoid selecting as SYNC fatheran endpoint whose reception rate indicator is too low, it is defined by:

${{SYNC\_ Penalty}{\_ RXI}} = \left\{ \begin{matrix}0 & {{{if}\mspace{14mu}{RXI}} \geq {RXI\_ Threshold}} \\{SP\_ RXI1} & {{{if}\mspace{14mu}{RXI}} < {{RXI\_ Threshold}\mspace{14mu}{AND}\mspace{14mu}{RSSI}} \geq {{- 100}\mspace{14mu}{dBm}}} \\{SP\_ RXI2} & {{{if}\mspace{14mu}{RXI}} < {{RXI\_ Threshold}\mspace{14mu}{AND}\mspace{14mu}{RSSI}} < {{- 100}\mspace{14mu}{dBm}}}\end{matrix} \right.$

The SYNC_Penalty_CD term introduces a preference for neighbors that havea better connectivity degree with the purpose of making the network morestable and reliable, it is defined by:

${{SYNC\_ Penalty}{\_ CD}} = \left\{ \begin{matrix}\infty & {{{if}\mspace{14mu}{CD}} = 0} \\{SP\_ CD1} & {{{if}\mspace{14mu}{CD}} = 1} \\0 & {{{if}\mspace{14mu}{CD}} \geq 2}\end{matrix} \right.$

The case CD=0 has been mentioned here for the sake of clarity. Aneighbor with CD=0 is not, by definition, a potential SYNC father andthe term will never be computed in that case.

Periodically, an endpoint will run the new SYNC father selection processto see if a better SYNC father has become available. These periodicselection processes should occur about once every hyperframe. They willbe implemented in such a way that different endpoints in a cell will runthe process at different times, thus avoiding too many endpoints sendinga SYNC request at the same time. A random time slot number could be usedfor this purpose. On the other hand, when an endpoint loses its SYNCfather, it should immediately begin the selection process for a new one.The selection process for this new SYNC father is described below.

The neighbor table will be analyzed to sort out the neighbors that matchthe potential SYNC father conditions. If endpoints belonging to othercells appear in this list, they should be removed from the list.Endpoints from other cells, when they are overheard, are dealt withaccording to the cell switching decision process described in thisdocument. The term SYNC_Merit will then be computed for all thepotential SYNC fathers in the list. The neighbor with the bestSYNC_Merit (lowest value) is called here X. The neighbor which had thebest SYNC_Merit at the time of the previous selection process is calledXP. If X is different from XP, a counter, SYNC_Top, is initialized tozero. If X is identical to XP, the counter SYNC_Top is incremented. If aneighbor manages to stay at the top of the potential SYNC father listfor SYNC_Top_N hyperframes or more, it is entitled to become the newSYNC father provided that this change brings an improvement ofSYNC_Merit larger than SYNC_Merit_Hyst1. At any rate, if choosing X asthe new SYNC father brings an improvement in term greater thanSYNC_Merit_Hyst2, the endpoint should select X as the new SYNC father. Adetailed step-by-step description of this selection process is givenbelow.

   Step 1: The neighbors are sorted according to the potential SYNCfather conditions to    make a list of potential SYNC fathers.       Ifthis list is empty, then       Discard all pending MAC messages and gointo discovery phase       Else if the list contains at least onepotential SYNC father       Go to step 2       End if    Step 2: Computethe term, SYNC_Merit, for each potential SYNC father.    Step 3: Thepotential SYNC father with the lowest SYNC_Merit value is identified(X). If the endpoint has lost its SYNC father, then    Step 4: Select Xas the new SYNC father (lowest SYNC_Merit). Go to step 7. If theendpoint still has its SYNC father, then    Step 5: SYNC fatherselection with temporal hysteresis according to the following   algorithm:       If X ≠ XP, then          SYNC_Top = 0          XP =X       Else if X = XP, then          SYNC_Top ← SYNC_Top +1         Comment: SYNC_Top is only incremented once every hyperframe.         If {[SYNC_Top ≧ SYNC_Top_N] AND          [SYNC_Merit(X) +SYNC_Merit_Hyst1 ≦ SYNC_Merit(SYNC father)]}, then             X isselected as new SYNC father             Go to step 7          Else            Keep the former SYNC father          End if    End if   Step 6: Look for a better SYNC father with the algorithm       IfSYNC_Merit(X) + SYNC_Merit_Hyst2 ≦ SYNC_Merit(SYNC father), then         Select X as new SYNC father       Else, keep the former SYNCfather       End if    Step 7: If a new SYNC father has been selected,send a SYNC request to X and wait for       acknowledgement (SYNCrequests are detailed in another part of this document).       If therequest is positively acknowledged, then          Stop (processcompleted)          Else if the request is not positively acknowledged(negatively       acknowledged or not acknowledged at all) and theendpoint has a valid SYNC       father             Abort the process         Else if the request is not positively acknowledged and theendpoint has no       more SYNC father             Go to step 1         End if

During step 7, if the endpoint decides to synchronize with a new father,it should:

-   -   Keep its MAC synchronization    -   Keep its level (in its header)    -   Refuse any synchronization request: send SYNC NACK    -   Refuse any message: send NACK (see traffic management)    -   Indicate in its messages that it is no longer synchronized by        setting the RS bit to zero.

Many Cell Masters (relays) can coexist in the field. These Cell Masterscan be part of the same network or they can belong to different networksor to different utilities. Affiliation of an endpoint to a network isindicated by the UID field in the MAC header and by the SFD value in thePHY header. An endpoint will never move to another network but it canswitch to another cell belonging to the same network. A utility caninstall additional Cell Masters in some areas in order to increase thedata throughput capability or to unburden a large cell. Additional CellMasters will also provide alternative routing paths that will contributeto the quality of service. To allow the traffic to be spread evenlyacross the available cells an endpoint which is already connected to acell should have the possibility to switch to another cell, with orwithout external intervention.

The method of manually asking a meter to switch to another cell is verysimple, if one of the endpoint belonging to the new cell is within radiorange. The user should only send a message to the endpoint that tells itthat its current cell is now forbidden and that the new one ispreferred. Then the endpoint will enter in a discovery phase to look forthe other cell and then synchronize with it.

An endpoint should also be able to switch to another cell automaticallyif it considers that it will have a better position in the new cell, andtherefore a better access to the WAN. This switching has to be done withsome precaution, because it can perturb an entire branch of the network.For this reason, the conditions to change to another cell should bestringent, particularly if everything works properly in the current one.

Before switching to another cell, an endpoint should of course know thatat least one representative of this other cell is in the neighborhood.As the physical layer is, by default, in listening mode, it happens fromtime to time that an endpoint receives a message from another cell.Indeed even if the hopping sequences are not the same, they are nevertotally orthogonal because they use the same set of channels and theyare not synchronized with the same time base. Occasionally, an endpointwill transmit a message when the channels of both cells are aligned, ifsome endpoints belonging to the other cell are within radio range, theywill hear the message. With only one message overheard from an adjacentcell, due to the parameters contained in the MAC header, an endpointwill have all the information to switch to that adjacent cell.

If the endpoint judges that the adjacent cell doesn't provide a betteraccess to the network, it will discard the information. If this accessis better, the endpoint can choose to resynchronize with the adjacentcell.

The criterion to declare that an endpoint from another cell is a betteraccess to the network is based on several parameters:

-   -   Low CSI. The smaller cell will be preferred to the larger one in        order to avoid having two adjacent cells with one full and the        other one empty.    -   Low GPD. The access with the smaller GPD will be preferred (the        value used here is the GPD between the endpoint and the Cell        Master through the neighbor, which is referred to as EP_GPD).    -   Low Level. A cell that provides an access with fewer hops to the        Cell Master will be preferred.

In order to make a selection based on a combination of the principlesmentioned above, we introduce a term to compare cells with each other.CELL_Merit=MAC_CELL_Weight_(GPDEP_GPD+MAC_CELL_Weight_Level*LVL++MAC_CELL_Weight_CSI*CSI

There are here defined three MAC layer parameters, MAC_CELL_Weight_GPD,MAC_CELL_Weight_Level and MAC_CELL_Weight_CSI. The values of theseparameters will depend on the importance one wants to give to GPD, tothe level or to the cell size in the cell switching decision process. Ifone sets the last two parameters to zero, only the GPD will be used tocompare cells. Upon overhearing a message from an adjacent cell theendpoint will compute the term, CELL_Merit for the new cell and also forits current cell. The condition for cell switching is: CELL_Merit (newcell)<CELL_Merit (current cell)−MAC_CELL_Switch_Hysteresis, where wehave introduced a new MAC layer parameter, MAC_CELL_Switch_Hysteresis,whose role is to prevent a node from continuously switching back andforth between two cells. Once the endpoint has determined that the newcell is better than the current one it has to make sure that it will beaccepted by the new cell before actually switching. For this purpose theendpoint will ask the endpoint from the other cell for synchronization.The SYNC Request and the SYNC (N)ACK will be exchanged as is done inother situations except that the endpoint needs to adjust its frequencyand time of transmission considering the other cell operates on adifferent hopping sequence.

Once the new father of the endpoint that leaves the cell answers a SYNCACK, the MAC layer needs to inform the layer above and stays in its oldcell during MAC_CELL_Leaving_Process_Duration seconds before definitelyleaves it. At that point, the different layers need to free theirbuffers and their pending actions. After the switch is done, the MAClayer informs the layer above again. This timing is necessary for theNET layer to deregister by sending a NET Cell Leaving Notificationmessage.

An example of a complete process of cell switching is as follows:

-   -   The endpoint overhears a message from an endpoint belonging to        another cell.    -   The endpoint saves the neighbor's parameters in its neighbor        table.    -   The endpoint checks if this neighbor meets the potential SYNC        father conditions. If not, the cell switching process is        aborted.    -   The endpoint computes the cell switching figures of merit for        its current cell and for the new cell. If the figures of merit        are favorable to the new cell, the endpoint goes on with the        cell switching process.    -   The endpoint sends a SYNC Request to the neighbor, on a forced        channel and in a forced sub-TS.    -   If the neighbor agrees and if it has a good connectivity with        its father, it then sends a SYNC acknowledgment, once again on a        forced channel and in a forced sub-TS. If the neighbor doesn't        agree it sends a negative acknowledgment and the cell switching        process is aborted. If the neighbor needs to check its        connectivity with its father, it will request a beacon and the        endpoint trying to switch cell will receive no immediate        acknowledgment.    -   If necessary, the endpoint will repeat its SYNC request until it        receives an acknowledgment or until the maximum number of        retries is reached. In this last case the cell switching process        is aborted.    -   Once the SYNC ACK is received, the endpoint dumps all messages        (at all layers) from its former cell.    -   The MAC layer informs the other layer and starts a timer with a        duration of MAC_CELL_Leaving_Process_Duration seconds.    -   Once the timer expires, the endpoint dumps all messages (at all        layers) from its former cell.    -   The endpoint synchronizes itself with the neighbor from the new        cell. (Be careful, the hyperframe number may have change during        transition period).

During this process, until the SYNC acknowledgement is received andduring the transition period, the endpoint should deal with its usualcommunication activities in its current cell.

-   -   Keep its MAC synchronization with the current cell    -   Keep its level    -   Don't initiate a cell switching with another cell, or a SYNC        father change with another endpoint.    -   Work as usual, keep other activities.

Yet another aspect of the present protocol subject matter advantageouslyrelates to a feedback control loop that may be used to correctimperfections of crystal clocks, and to maintain synchronization in afrequency hopping spread spectrum (FHSS) mesh network.

As otherwise discussed herein, the present protocol is based onfrequency hopping spread spectrum (FHSS) for better interferenceimmunity and to be in compliance with the radio regulations in somecountries. In a FHSS system per the present subject matter, all thenodes hop their channel frequency according to the same pseudo-randomsequence in order to be perfectly synchronized for both reception andtransmission. The performance of such a system is based on the abilityfor each node to be able to maintain such form of synchronization overtime. That is the reason why the node hardware requires a crystal timereference with good stability. Because such time references areexpensive, it is useful to implement a software driven compensationmechanism to improve the time stability of the nodes.

In the present exemplary network, as otherwise discussed herein, amesh-like structure is provided wherein the cell relay is the root ofthe mesh and the metronome of the network. As a rule, such timinginformation propagated away from the root to the cell nodes. In thepresent protocol subject matter, each time a node communicates withanother node closer to the root, it will realign its clock to be insynchronization with the network. In addition, all such consecutiveclock corrections advantageously are time averaged, filtered, andprocessed to give information about how fast the node's clock is runningwith respect to the average clock of the nodes closer to the root. Suchpresent feature allows a software correction of the node's clock rhythmthat will bring it in tune with the network for a long period of time.Therefore, generally speaking, the present subject matter providesbenefits of allowing the usage of relatively low cost crystals in thenetwork nodes but with increased time stability of the network.

More specifically, present FIG. 27 illustrates a typical distribution ofresynchronizations and crystal drift corrections in time, per thepresent subject matter.

As otherwise referenced herein, good synchronization is the basis of thepresent protocol, wherefore an inherent limitation to that aspect wouldotherwise come mainly from the crystal accuracy. In order to limit thetraffic and to avoid internal collisions, as few synchronization beaconsas possible are sent. However, as a result, in low traffic conditions,the number of opportunities to resynchronize the clock with a fatherwill therefore be relatively small. As a consequence, each endpointwould generally have a clock shift with the upper level. For relativelylarger level numbers, such shift would become significant relative tothe cell relay clock. This could lead to a loss of synchronization ifthe shift were allowed to grow above some limit. Moreover, as anendpoint can resynchronize its clock with several father endpoints, anaveraging mechanism could be advantageously utilized.

One approach of the present subject matter as a solution is toanticipate the drift of the local crystal oscillator with respect to thefather clocks. If such drift is assumed to be constant (shown to be agood assumption if the temperature doesn't change too quickly), anefficient compensation procedure can be implemented. Therefore, ratherthan waiting for the next resynchronization, the endpoint can adjust itstime slot length to decrease the next clock resynchronization value. Thecompensation algorithm uses low-pass filtering to cope with multiplesynchronization paths and to avoid instabilities. The detaileddescription of such algorithm is otherwise discussed herein.

Whenever the receiver resynchronizes its local clock, two values arerecorded: the value of the correction, which is Clock_Correction(k), andthe time of this resynchronization, which is Resync_Time(k). This timeis given by the value of the system real-time-clock at the moment of theresynchronization. The parameter k is used here to number the successiveresynchronization occurrences. From these two values and with theknowledge of the previous resynchronization time, it is theoreticallypossible to evaluate the relative drift of the local crystal oscillator,Xdrift. To be useful for the purpose of crystal drift compensation,these evaluations must be accurate.

Each clock correction value can be considered to be the result of twocontributions. The first one is a slow drift due to a difference betweenthe local crystal frequency and the average crystal frequency in thefather endpoints. The second contribution is a random time shiftoccurring each time a packet travel time is estimated. This issummarized in the following equation:Clock_Correction(k)=Xdrift[Resync_Time(k)−Resync_Time(k−1)]++δt(k)−δt(k−1)

where δt(k) is a random error due to the uncertainty in the propagationtime of the packet from the transmitter MAC layer to the receiver MAClayer when resynchronization number k is performed.

To reduce the contribution of random errors, successive clockcorrections are preferably summed, as follows:

Clock_Correction(k) + Clock_Correction(k + 1) = Xdrift[Resync_Time(k + 1) − Resync_Time(k − 1)] + δ t(k + 1) − δ t(k − 1)

It is readily understood per the present disclosure that, in theevaluation of the crystal drift, the contribution of these random errorswill become increasingly smaller as successive clock corrections aresummed, as follows:

${Xdrift} = \frac{\left\lbrack {\sum\limits_{m = k}^{k + M}{{Clock\_ Correction}(m)}} \right\rbrack + {\delta\;{t\left( {k + M} \right)}} - {\delta\;{t\left( {k - 1} \right)}}}{{{Resync\_ Time}\left( {k + M} \right)} - {{Resync\_ Time}\left( {k - 1} \right)}}$

For this reason, the successive clock correction values are summed upuntil they span a time larger than the minimum value specified by theMAC layer parameter MAC_Xdrift_Tmin. Once this time value is exceededthe crystal drift can be evaluated with the following:

${{Xdrift}(n)} = {\frac{\sum\limits_{k = {{k{({n - 1})}} + 1}}^{k{(n)}}{{Clock\_ Correction}(k)}}{{{Resync\_ Time}\left( {k(n)} \right)} - {{Resync\_ Time}\left( {k\left( {n - 1} \right)} \right)}}.}$

The summation range should respect the condition:Resync_Time(k(n))−Resync_Time(k(n−1))>MAC _(—) Xdrift_(—) Tmin

where MAC_Xdrift_Tmin is chosen large enough to have:

$\frac{2\max{{\delta\;{t(k)}}}}{{MAC\_ Xdrift}{\_ Tmin}} < {{Xdrift\_ accuracy}.}$

Xdrift_accuracy is the targeted accuracy of the algorithm (about 1 ppm).MAC_Xdrift_Tmin must also be much larger than the interval between twoleap time slots (as otherwise discussed herein) in order for the timeintegration process to smooth out the crystal drift compensationglitches.

Present FIG. 28 represents in schematic format a local clock driftcompensation algorithm for practice per the present protocol subjectmatter, while present FIG. 29 represents (also in schematic format) alow-pass filter for crystal drift correction, all in accordance with thepresent subject matter.

The herein referenced estimation regarding the reference drift will notbe used directly to compensate for the crystal oscillator drift. Inorder to average over several synchronization sources and to get rid ofthe fluctuations, a low-pass filter (see present FIG. 29 representation)will be implemented. This low-pass filter is defined by:Xdrift_filt(n)=A×Xdrift(n)+B×Xdrift_filt(n−1),where Xdrift_filt(n) is the filtered crystal drift estimation and A,Bare the filter coefficients that will be adjusted to obtain adequateaveraging and to make the resultant feedback control loop stable enough.These two filter coefficients will have values given by the followingMAC layer parameters:A≡MAC_Xdrift_Filter_AB≡MAC_Xdrift_Filter_BThe instantaneous length of the system time slot T_(slot)(n) is defined,and such value can be expressed as the sum of the default time slotlength and a small correction term:T _(slot)(n)=TS_Length+ΔT _(slot)(n)The instantaneous value of the time slot length is updated by:T _(slot)(n)=T _(slot)(n−1)(1+Xdrift_filt(n)).As the correction is expected to be very small, one can neglect thesecond order term. The simplified version is:T _(slot)(n)=T _(slot)(n−1)+TS_Length×Xdrift_filt(n)In practice, generally only the time slot length deviation needs to becomputed:ΔT _(slot)(n)=T _(slot)(n−1)+TS_Length×Xdrift_filt(n)−TS_LengthThis can be expressed as a function of the previous deviation:ΔT _(slot)(n)=ΔT _(slot)(n−1)+TS_Length×Xdrift_filt(n)

In order to make sure that the mathematical description of the algorithmrepresented by present FIG. 28 is well understood, the whole process ishere summarized with pseudo-code.

First Initialization

Xdrift_filt = 0 Start_Time = first resynchronization time valueSum_Clock_Corr = 0 Upon reception of a beacon or any valid message do    Accumulate the clock correction     Sum_Clock_Corr =Sum_Clock_Corr + Clock_Correction     Update the time since the lastcrystal drift estimate Time_Since_Last_Xdrift_Update = Resync_Time −Start_Time     If Time_Since_Last_Xdrift_Update < MAC_Xdrift_Tmin       Then wait for next message        Else do           Compute thecrystal drift Xdrift = Sum_Clock_Corr / Time_Since_Last_Xdrift_UpdateFilter the drift estimate Xdrift_filt=A * Xdrift + B * Xdrift_filtUpdate the time slot correction ΔTslot = ΔTslot + Xdrift_filt *TS_Length Initialize Start_Time for the next iteration          Start_Time = last value of Resync_Time           Initializethe clock correction accumulator for the           next iteration          Sum_Clock_Corr = 0           Wait for a new message     End

The accuracy required for proper crystal drift compensation is about 1ppm. This will probably make a direct correction of the parameterMAC_TS_Length impossible, especially if the resolution of the SACT isnot very high. Therefore, it is suggested, at the beginning of each timeslot, to reload the countdown timer with the default time slot lengthvalue, MAC_TS_Length. Every MAC_Xdrift_LeapTS time slots, a “leap timeslot” will be introduced. This is explained with the followingpseudo-code:

If Time_Slot_Number = 0 (modulo MAC_Xdrift_LeapTS) Then Count_Down_Timer← MAC_TS_Length + MAC_Xdrift_LeapTS * ΔTslot Else Count_Down_Timer ←MAC_TS_Length End

In the above code, Time_Slot_Number is a number starting from 0 andincremented at each time slot, it is not the time slot number used toidentify the position within a hyperframe. It should be pointed out thatfor optimum crystal drift compensation, the above SACT reload should beperformed with the full accuracy provided by the firmware, as specifiedby the parameter MAC_FW_Acccuracy. The resolution of the correctionalgorithm depends on the leap time slot interval as shown by thefollowing equation:

${{Crystal}\mspace{14mu}{drift}\mspace{14mu}{correction}\mspace{14mu}{resolution}} = \frac{{LSB}\mspace{14mu}{of}\mspace{14mu}{SACT}}{{MAC\_ Xdrift}{\_ LeapTS}*{MAC\_ TS}{\_ Length}}$

A resolution better or equal to 1 ppm should be targeted. On the otherhand, the leap time slot interval should be small enough to avoid clockcorrections larger than the time slot margin. In satisfying suchcriteria, the following inequality should be respected:

${MAC\_ Clock}{\_ Accuracy}\mspace{11mu}\text{<<}\mspace{11mu}\frac{{MAC\_ TS}{\_ Margin}}{2*{MAC\_ Xdrift}{\_ LeapTS}*{MAC\_ TS}{\_ Length}}$

Part of the advantages of the present protocol subject matter is theprovision of a system which itself is based on a self management andoptimization system of endpoints that is organized into cells. Each cellhas a Cell relay which serves the purpose of relaying all theinformation to and from the network to another wide area networkoperating on a TCPIP protocol. Due to such fact, the assimilation of anendpoint to a given cell is uncontrolled and may grow unbounded. Ofcourse, Cell relays have resource limitations and growing beyond certainlimits will overload these resources.

The present particular aspect of the present subject matter is based oncertain indicators of the cell size that is communicated to allendpoints in the cell. Endpoints that are joining the network and couldhave the possibility to join more than one cell will use thisinformation in the decision process of which cell to join. If theindicators are that cell A is full or close to full, cell B will bechosen per present subject matter to synchronize with even if theindications are that cell A may be a much better cell for uploadingnetwork traffic.

Although cells operate in isolation due to the quasi orthogonalfrequency hopping sequences, on rare occasion's traffic will beoverheard from one cell to the other for the endpoints located on thetouching boundaries. In these events, the cell size indicators can beused to drive a decision to migrate from a full cell to an empty or muchless full cell. Accordingly, per present subject matter otherwisediscussed herein, based on such present processes, the cell sizes willbe managed and balanced over time, allowing self management andoptimization to continue.

More particularly, present aspects of this subject matter are applicablefor embodiments configured as a tree network where the nodes areorganized in cells with a concentrator (or root node) at the “head” ofeach cell. As otherwise discussed herein, each node has a levelassociated with its place in the cell. The root node is level 1 and isalways synchronized (by definition). In order to get its data to theroot node a node must be synchronized in the corresponding cell. Thesynchronization process requires a hand check between one synchronizednode and another node. A node which synchronizes on another becomes itsson and the other node becomes the father of the first one. The new nodelevel is one more than the one of its father. Therefore, all nodes havea level above 1. Nodes of the same level are called brothers. The groupof fathers, brothers and sons of a node are called its neighbors.

Per present subject matter, each node keeps a table of its neighbors.The information on the neighbors is used for several purposes(synchronization, routing, and the like), including the transmission ofbroadcast packets. Effectively, the broadcast is actually a multicastmessage sent to all the sons of the node. It is thus important that eachnode is in the neighbor table of at least one of its fathers to insurethe delivery of broadcast packets. This is one of the roles of thesynchronization request: by sending a synchronization acknowledgement(SYNC_ACK), a father guarantees that its new son is in its neighbortable. Per present protocol, the father of a node which sent a SYNC_ACKis called the SYNC_FATHER of this node. The SYNC_FATHER is the onlyfather which guarantees that the node is in its neighbor table, and thusthe only father which guarantees it will send a broadcast to the node. Anode must always have one SYNC_FATHER.

Whenever the memory of a node is limited, likewise so is its neighbortable. The technical problem becomes when the table is full and a newnode requests synchronization. The synchronized node with the full tablecannot acknowledge positively the synchronization request of the newnode without inserting it in the table. Any such activity could lead toa node in the cell not receiving broadcasts (if it is not in the tableof another father). Unfortunately, if the synchronization right isrefused, then it could lead to an orphan node (not in a cell) not beingable to deliver its data. In the same way, the “father” node cannot makespace for the new node by removing any of the neighbors in its table(because to do so could cause a node to not receive broadcasts).

The solution achieved by the present management protocol is primarilybased on two things. First, a bit (EPSF for Enough Potential SyncFather) is sent in each packet and kept in the neighbor table for eachneighbor. This bit is set to 1 by each node if the number of father andbrothers in its neighbor table is above a given threshold (which ischosen to indicate that they could safely send a request to anothernode). The second part is the table out notification (TON) message.Based on the EPSF bit, a node receiving a new synchronization requestwhile its neighbor table is full, can decide to remove one of its sonsif this son's EPSF bit is one. But it must indicate to this son that itwill no longer be in its neighbor table. Such is accomplished by sendingthe TON message to this son. Upon receiving this message, this son willlook if this father was its SYNC_FATHER. If it was the case, then itmust find another SYNC_FATHER to guarantee that it will be in theneighbor table of one of its fathers and thus receive broadcasts.

This solution provides a way to never refuse synchronization to a newnode while making sure that all nodes are in the neighbor table of oneof their fathers, and thus making sure that they will receive broadcastpackets.

When it comes to neighborhood management and neighborhood informationper the present protocol subject matter, the MAC layer is in charge ofmanagement vis-à-vis neighbors. Accordingly, each time an endpointreceives a message, it also saves or updates the parameter of the senderon a list. Therefore, per present subject matter, only 1-hop neighborparameters are known and saved in the endpoint. The Cell relay is theonly device that knows status of the endpoints belonging to its cell,but it is managed with the neighbor list in the network layer.

The MAC layer not only will manage its own neighbor table but will alsoindicate to the upper layers (particularly to the NET layer, by way ofthe LLC layer) some of the changes when they occur (for example,inclusion of a new neighbor).

The MAC Neighbor table contains parameters of the neighbors. It islimited in size by the variable Max_Nb_of Neighbors. For each neighborthe parameters are:

-   -   Address, 4 bytes (The MAC Address of the neighbor).    -   Level, 1 byte (The last known level of the neighbor. The level        is 0 if the neighbor is not synchronized. Level 1 is the cell        relay).    -   Average RSSI, 1 byte (RSSI is measured during the reception and        indicated by the physical layer. The saved value is an average        value of RSSI over a sliding window with this neighbor).    -   Cell Relay Address, 2 bytes (The cell to which the neighbor        belongs).    -   Last TS (Time Slot) number, 2 bytes (The time slot where the        last reception occurred. With the channel sequence information,        it gives the last channel).    -   Last Reception Time, 4 bytes (The time where the last reception        occurred. The reference of this time is free to the        implementation. It is however suggested to be the power-up time        of the node.).    -   Delta SACT, ΔSACT, 2 bytes (The SACT difference between the EP        and this neighbor. This value is refreshed each time a message        is received from this neighbor. This indicates the misalignment        between the 2 EPs).    -   GPD, 2 bytes (The Global average Propagation Delay between the        neighbor and the cell relay. This value is indicated in the MAC        header).    -   LPD, 1 byte (The Local average Propagation Delay between the EP        and the neighbor).    -   Last reception rate update (Last time the reception rate of this        EP was updated).    -   Received rate indicator, RXI, 1 byte (It provides an indication        of the frequency the endpoint receives messages from this        neighbor. It will prevent the endpoint from synchronizing with a        neighbor that is barely heard).    -   Sync_Forbidden, 1 bit (A flag indicating that this endpoint has        refused synchronization).    -   Sync_Father, 1 bit (A flag indicating that this neighbor allowed        the endpoint to get synchronized. Only one neighbor can have        this bit set. It is set when the SYNC ACK is received. When an        endpoint changes its level, it should clear all flags except the        ones assigned to the neighbor it just synchronized with. An        endpoint should have a father with the SYNC Father bit set in        order to consider itself synchronized).    -   Sync_Son, 1 bit (A flag indicating that the EP has allowed this        neighbor to be synchronized with it. This flag should be removed        if the level of this neighbor changes).    -   Registered, 1 bit (A flag indicating that this neighbor is        registered to a cell).    -   Enough_Potential_Sync_Fathers, 1 bit (A flag indicating if this        neighbor has a number of brothers and fathers that it can        Synchronize with (Sync_Forbidden bit=0), that is greater than        MAC_Good_Number_of_Potential_Sync_Fathers).    -   Father Acknowledged load, (S_(Ack)) 1 byte (Average number of        acknowledgements received from this father excluding the        acknowledgments intended for the EP itself. This is maintained        only if the EP is a father. This is used to estimate the load of        this father, which in turn will be used to determine the        randomization window for transmission).    -   CSI, 2 bits (The Cell Size Indicator of this neighbor).

Due to memory limitations, the neighbor table has a finite size andcannot contain more than Max_Nb_of_Neighbors entries. It is thereforenecessary to get rid of some entries as they become useless for theoperation of the network.

The neighbor table is managed according to the following generalprinciples:

-   -   Only neighbors that satisfy the potential SYNC father conditions        will be added to the table.    -   As long as the table is not full, any new neighbor that        satisfies the potential SYNC father conditions will be added to        the table.    -   If the table is full when a new potential SYNC father arrives,        the new neighbor will take the place of a less important one, if        such a neighbor exits. The importance of a node in the neighbor        table is based on the synchronization term. If there is no        possibility to free some space in the table, the information on        that new neighbor will be discarded.    -   Nodes from which nothing has been received for a long period of        time will be considered to have left the 1-hop neighborhood. If        no message has been received from a neighbor for a period of        time longer than MAC_Neighbor_Timeout, the neighbor will be        removed from the neighbor table.    -   If an endpoint goes back in discovery phase, switches to another        cell or experiences a power outage, all the entries of the        neighbor table should be deleted.

The process of freeing up space in the table can be further detailed asfollows. It should be pointed out that this process is not carried outon an ongoing basis but only when a new potential SYNC father needs tobe inserted in a table which is already full.

-   -   Check the potential SYNC father conditions. If some entries do        not match anymore the potential SYNC father conditions, they can        be deleted from the table.    -   The most important neighbors in the table are the ones with the        lower synchronization term. If a node needs to be taken out, the        one with the highest term should be removed (see exception for        SYNC father below).    -   Of course if the new neighbor has a worse term than the worst        node in the table, the new neighbor is discarded.

If an endpoint is not synchronized, any neighbor that match thepotential SYNC father criterions can be added to its neighbor table. Therelative importance of these entries will be defined according to theterm for the discovery phase, SYNC_Disc_Merit.

If the endpoint is in a cold start process, only the neighbors belongingto the preferred cell will be allowed into the neighbor table. If theendpoint is in a warm start process, all potential SYNC fathers,whatever the cell they belong to, will be allowed into the table.

If an endpoint is synchronized, the importance of the entries is basedon the synchronization term (SYNC_Merit). If a node needs to be takenout, the one with the highest SYNC_Merit should be removed. There is oneexception to this rule: the SYNC father cannot be removed from thetable. If a father needs to be removed when the SYNC father happens tohave the worst SYNC_Merit, the next-to-worst should be removed.

The synchronization term depends, among other parameters, on thereception rate indicator, RXI. As newcomers have a low RXI, this willcreate a hysteresis for the inclusion of new neighbors in the table.This will limit the coming and going in the table.

In synchronized mode, nodes from other cells are not entered into thetable. They are evaluated on the fly to check if they could offer abetter synchronization point.

Present FIG. 30A also depicts in flowchart format present neighbor tablemanagement.

Endpoints have fixed MAC addresses and can potentially synchronize andconnect to any cell. However, the protocol should take into account thatsome cells are forbidden. This can be the case if the user/utility wantsto control the repartition of endpoints in different cells, or merely ifa user doesn't want to share its network with another user/utility (thislast case is normally handled by having different Utility IDs in the PHYheader). In order to manage the membership of a cell, a Cell Addressuniquely identifies each cell.

From the MAC layer standpoint, a node can be synchronized with any cell.It is therefore the role of the API layer to tell the MAC layer whethera cell is authorized or not. This information is then kept at the MAClayer level, which will not consider a forbidden cell forsynchronization.

All the forbidden cells are re-authorized by the MAC layer if theendpoint stays non-synchronized for a period of time longer thanMAC_Unsynchronized_Timeout.

In cold start, once the node is synchronized, it informs the API layerof this successful synchronization indicating the corresponding celladdress. The API layer should then inform the MAC layer when theendpoint becomes registered. The MAC layer will not authorize othernodes to synchronize with itself if it is not registered. As soon a thenode is registered, the MAC layer will save the cell address and use itas preferred cell in case of warm start.

In warm start, the node looks for its preferred cell. If it succeeds tofind the cell and synchronize with one of its members, the MAC layerwill consider itself already registered (RS bit=1), and will immediatelyauthorize the synchronization requests of its neighbors. The API layerneeds to tell the MAC layer whether this assumption was correct or not.

The warm start will accelerate the synchronization process of a cellafter a large-scale power outage.

In general, per the present protocol subject matter, two kinds ofmessages are acknowledged: Monocast Data containing LPDU from LLC layer,and SYNC requests. Monocast Data are acknowledged (or not) with ACK (orNACK) messages and SYNC requests by SYNC ACK (or SYNC NACK) messages.

ACK, NACK, SYNC ACK and SYNC NACK must be sent in the time slot of thereception of the corresponding packet. More precisely, theacknowledgement should be sent in the last sub-time slot.

Each message that should be acknowledged has a MAC frame ID (FID),inserted in the MAC header. The (non-) acknowledgment message willmention this frame ID in its own MAC header. The MAC frame ID is acounter, incremented by the MAC layer at each sending of Monocast Dataor SYNC request.

For each LPDU the LLC asked to send, the MAC layer will indicate back ifthe Monocast Data message has been acknowledged (ACK), non-acknowledged(NACK), or not acknowledged (neither ACK, nor NACK received).

The Broadcast Data messages are not acknowledged. They are not addressedto any node in particular and thus contain no destination address intheir MAC header, or MAC frame ID. When the Broadcast Data message hasbeen sent, the MAC layer will notify it to the LLC layer.

The present protocol subject matter advantageously uses a 32-bit CRC(Cyclic Redundancy Check) to avoid message corruption by either of noiseor interference. The CRC is computed by the sender on the entire MACheader and LPDU and placed at the end of the frame. On the receiverside, the value of the CRC is used to verify the message validity. Ifthe CRC matches the message, the frame is accepted. If it does notmatch, it is discarded.

The CRC used is the standard 32-bit IEEE 802.3 CRC. Present FIG. 30provides a schematic representation of a typical CRC 32-bitimplementation. The generator polynomial of such CRC is:G(x)=x ³² +x ²⁶ +x ²³ +x ²² +x ¹⁶ +x ¹² +x ¹¹ +x ¹⁰ +x ⁸ +x ⁷ +x ⁵ +x ⁴+x ² +x+1

The CRC is computed with a linear feedback shift register initialized to0xFFFFFFFF (or any equivalent implementation). The computation beginswith the first byte of the MAC header and ends with the last byte of theLPDU. Each byte is fed into the shift register with the leastsignificant bit first. At the end of the polynomial division, the shiftregister contains the remainder of the division. The first byte to beshifted out of this register corresponds to the first redundancy byte.It is interpreted least significant bit first and should be complementedto one before being appended to the LPDU.

With reference to security, the present protocol subject matterpreferably does not provide any encryption service. As such, thedatagrams are preferably sent on the air interface without encryption.However, that is not to say that the present protocol subject matter isnot a secure protocol. It is in fact a designed protocol, the physicallayer for which uses a FHSS technique with a very long frequency-hoppingpattern. Eavesdropping on such a system would require a significantengineering effort. This intrinsic security is further enhanced by theuse of Fibonacci sequences to make the hopping pattern different in eachcell.

Should such approach to security be considered insufficient for somecritical applications, it is within the scope of the present subjectmatter to supplement such security by encrypting the user data in theapplicative layers.

Certain advantageous aspects of the present subject matter relate towhat may be regarded generally as network traffic regulation, or morespecifically as network traffic load control. In particular, proceduresare provided to avoid conditions under which the traffic load growsabove a point of gridlock in a slotted Aloha mesh network. In certainaspects, the present procedures use the monitoring of receivedacknowledgements to evaluate the traffic density. At least severalidentifiable benefits of such present methodologies are that it allowsthe uplink traffic in a mesh network to flow in optimal conditions, andit avoids traffic gridlock due to operation of the network beyond itslimit.

Per present subject matter, traffic load control is used to limittraffic in order to avoid using the channel beyond its optimal trafficdensity. This is necessary because the present protocol subject matteroperates as a slotted Aloha system, and for such a system, trafficdensity above some limit can increase the collision rate to anunacceptable level and completely block the data flow (that is, dataflow becomes gridlocked). The present traffic control preferably is usedonly for the upload traffic, from endpoints to the cell relay (or CellMaster).

More particularly, the present traffic load control subject matter isapplicable to generally any mesh network with a node acting as a dataextraction point. The data traffic from the individual nodes to thisextraction point is regarded as the uplink traffic. As such uplinktraffic generated in the network grows higher, internal collisionsbetween packets will occur. At some point, such internal collisions willbe frequent enough to degrade the effective throughput of the system.The relation between collision probability and effective throughput iswell known from slotted Aloha theory. Textbook theory deals with thecase where no external jammer is present. Here the situation is moredifficult to analyze because there are both types of collisions at thesame time, that is, internal collisions due to internal traffic, andexternal collisions with the other users of the band.

Accordingly, the present subject matter advantageously introduces acontrol mechanism to slow down the data traffic when it grows above somelimit. The nodes need to be able to temporarily hold off transmissionsand store messages in a buffer when they detect that the channel is toobusy. This traffic load control will prevent the nodes from using thechannel beyond its optimal traffic density. If this is not done, thetraffic density can increase the collision rate to an unacceptable levelthat will not only decrease the performance but that may completelyblock the data flow.

Therefore, per the present subject matter, in order to control properlythe traffic load, an endpoint needs to evaluate the amount of trafficgoing through the network. For present description purposes, a firstnode within the radio range of a second node and in the direction of theextraction point for the second node, will be called a father noderelative to such second node. Present FIG. 31 is a schematicrepresentation of the present subject matter traffic load monitoring,where a given node B is listening to the (N)ACK messages from itsfathers A and C. For the afore-mentioned traffic load control purposes,an exemplary endpoint (node B in exemplary FIG. 31) will record theacknowledgments transmitted by its fathers A and C and not intended foritself. Such acknowledgments will give enough information to assess thetraffic load because in the present protocol, a node has to acknowledgeall the data packets it receives. This approach is consistent with thefact that traffic management will be mainly used by the endpointsdirectly communicating with a cell relay, from which onlyacknowledgments are transmitted in an upload situation. However, suchinformation is nevertheless not enough because a node needs to be ableto distinguish between a low traffic situation that generates only a fewacknowledgements and a very high traffic situation that also generatesfew acknowledgements because most of the packets are lost due tocollisions. For this purpose, preferably each node will record all thecommunication attempts with the father nodes and will compute an averagecommunication success rate. Combining both the rate of acknowledgmentsoverheard and the communication success rate, a node will be able toestimate the traffic density in an unambiguous way.

In a formal way, one can define the traffic input density R_(A) as theaverage number of data packets arriving at node A (FIG. 31) in one timeslot. This concept is useful to measure how busy is node A. It is alsoknown from slotted Aloha theory that the traffic input density has anoptimal value. If the traffic input density grows above that optimalvalue, the throughput goes down due to collisions. All the data packetsarriving at node A are considered in the definition of traffic inputdensity, whether they are successfully received or not. However, forpractical reasons, packets emanating from node B are preferably excluded(since the focus is currently on trying to derive a behavior rule forsuch node B). Also defined are the average number of acknowledgmentsemanating form node A and overheard by node B (excluding those intendedfor node B) in one time slot, S_(A). The communication success rate fromnode B to node A is called CSR_(BA) and Q_(B) is defined as theprobability for node B to be in listening state. From elementaryprobability theory it can be shown that the estimation for the trafficinput density at node A is given by the following:

$R_{A} = \frac{S_{A}}{{CSR}_{BA}Q_{B}}$

To avoid overflowing a node with packets beyond the optimal trafficinput density, the transmission rate of packets is limited per thepresent subject matter. For this, there is defined a maximum trafficinput density, R_(MAX). From slotted Aloha theory, this should be equalto one, but to be conservative by design, it is preferable to use asmaller value. The total traffic input density at node A is the sum ofthe estimated traffic density R_(A) and the traffic that node B willgenerate for node A. Node B will modulate the traffic it generates fornode A in order to prevent the total traffic input density at node Afrom exceeding the maximum allowed value R_(MAX).

A straightforward way per present subject matter to implement thislimitation is to send the messages at a random time within arandomization window of length T_(W). The length of the randomizationwindow should respect the following condition:

${{\frac{1}{T_{W}} + R_{A}} \leq R_{MAX}},$where T_(W) is expressed in time slot units.

Per present subject matter, if a node has several fathers, preferably itshould size the length of its randomization window according to thefather with the highest traffic input density, even if the packet is notintended for this father.

The following tasks preferably are performed in every node of thenetwork. They are to monitor all acknowledgments overheard from thefather nodes; to record all the communication successes/failures withevery father node; to keep a record of the time spent in transmission orlistening state; to compute the probability for the receiver to belistening, Q; for every father node, to compute the communicationsuccess rate CSR and to compute the estimated traffic input density, R;and to slow down of hold off transmissions if the traffic input densityfor the busiest father becomes too large.

All such average variables (input traffic density, rate of overheardacknowledgments, communication success rate and probability for a nodeto be listening) can be computed with a sliding average algorithm toavoid using excessive microprocessor memory.

Referring to such subject matter in somewhat different terms, perpresent subject matter, a defined maximum traffic input density may bereferred to as MAC_Traffic_Density_max, such that the total trafficinput density at endpoint A, now including the traffic from B to A isgiven by:

${\frac{1}{Tx\_ Window} + R_{A}} = {{MAC\_ Traffic}{\_ Density}{\_ max}}$Where Tx_Window is the length in time slots of the randomization windowused to transmit a packet. The data packet will be transmitted in arandomly chosen time slot within this randomization window. It followsan equation to compute the length of this window as a function of threereadily measured parameters:

${TX\_ Window} = \frac{1}{{{MAC\_ Traffic}{\_ Density}{\_ max}} - R_{A}}$with R_(A)=S_(ackA)(1+LPD_(BA))/Q_(B)where LPD_(BA) is the local propagation delay from B to A, Q_(B) is theprobability for endpoint B to be in receiving state and S_(ackA) is theaverage number of acknowledgments transmitted by A and received by B bytime slot. For practical purposes, the length of Tx_Window needs to bebounded. The outcome of this calculation will be truncated in order toalways be in the following range:MAC_Tx_Window_min≦Tx_Window≦MAC_Tx_Window_max.The randomization window length will then be computed with thefollowing:

${Tx\_ Window} = \left\{ \begin{matrix}{{round}\left\lbrack {\min\left( {\frac{1}{R_{A\;\max} - R_{A}},{{MAC\_ Tx}{\_ Window}{\_ max}}} \right)} \right\rbrack} & {{{if}\mspace{14mu} R_{A}} < R_{A\;\max}} \\{{MAC\_ Tx}{\_ Window}{\_ max}} & {{{if}\mspace{14mu} R_{A}} \geq R_{A\;\max}}\end{matrix} \right.$where it is used R_(Amax)@MAC_Traffic_Density_max. The endpoint has tomonitor the traffic for each one of its fathers in order to have anup-to-date value of S_(ackA). At the end of each time slot, the endpointcomputes new values of the S_(ackA) parameters in its table ofneighbors. This has to be done systematically, whether a packet fromthat neighbor was received or not. We use for this a sliding averagewindow as defined below:

${S_{ackA}(n)} = {{\frac{N_{TMW} - 1}{N_{TMW}}{S_{ackA}\left( {n - 1} \right)}} + \left\{ \begin{matrix}0 & {{if}\mspace{14mu}{no}\mspace{14mu}(N){ACK}\mspace{14mu}{is}\mspace{14mu}{received}\mspace{14mu}{from}\mspace{14mu} A} \\\frac{1}{N_{TMW}} & {{if}\mspace{14mu} a\mspace{14mu}(N){ACK}\mspace{14mu}{is}\mspace{14mu}{received}\mspace{14mu}{from}\mspace{14mu} A}\end{matrix} \right.}$In that formula n refers to the time slot number and N_(TMW) is thenumber of time slots in the traffic monitoring window. This number isgiven by the MAC layer parameter N_(TMW)@MAC_Traffic_Monitoring_Window.Q_(B) is also updated at each time slot with the following:

${Q_{B}(n)} = {{\frac{N_{TMW} - 1}{N_{TMW}}{Q_{B}\left( {n - 1} \right)}} + \left\{ \begin{matrix}0 & {{if}\mspace{14mu}{endpoint}{\mspace{11mu}\;}{in}\mspace{14mu}{Tx}\mspace{14mu}{mode}} \\\frac{1}{N_{TMW}} & {{if}\mspace{14mu}{endpoint}{\mspace{11mu}\;}{in}\mspace{14mu}{Rx}\mspace{14mu}{mode}}\end{matrix} \right.}$

Of course, if an endpoint has several fathers, it should always size thelength of its randomization window according to the father with thehighest traffic input density, even if the packet is not intended forthis father.

Due to hardware cost, the memory size to save messages will not beunlimited from a system point of view. Packets during their travelbetween endpoint and Cell Master are buffered into nodes. To avoidstanding in front of irresolvable blocked traffic situations, whenmemory is full, packets storage should follow some important guidelines.

Packets storage should be divided into two categories:

-   -   Packets going uplink: Uplink, Broken Link, Outage Notification .        . .    -   Packets going downlink: Downlink, Broadcast . . .

The number of packets belonging to each category should be monitoredover time and is called nb_of_uplink_buffered_packets andnb_of_downlink_buffered_packets. There is a maximum number of packetsthat can be saved for each category.

-   -   nb_of_uplink_buffered_packets≦MAC_Max_nb_of_uplink_buffered_packets        nb_of_downlink_buffered_packets≦MAC_Max_nb_of_downlink_buffered_packets        nb_of_uplink_buffered_packets+nb_of_downlink_buffered_packets≦memory        size

To maintain this information it is necessary that the different layersindicate the category of the packet they send/receive. As the CellMaster only receives uplink traffic and sends downlink traffic, thesecategories can be respectively compared to ingoing and outgoing bufferedpackets.

Once a buffer of either type is full, if a message of the correspondingtype of is received, the MAC layer should respond to the sender with aNACK message and discard the packet as there is no place to save it.

For the Cell Master case, if the WAN connectivity is good, the uplink(ingoing) buffer should in theory never be full. Indeed the throughputof the WAN is highly superior to the Linknet one. If the uplink bufferhappens to be full, the same algorithm is used and the Cell Masterstarts sending NACK to incoming packets. This situation will incounterpart highly degrade network performances and can bring aboutnetwork instability and packets losses.

With respect to the following discussion points on the presentscheduling of messages, it should be understood that in this context amessage refers to any packet other than an acknowledgement. When theapplication layer requests it, or when there is a received message toforward, the NET layer determines the destination address(es). The LLClayer deals with the fragmentation and retransmission of messages. Thesetwo layers send requests to the MAC layer who adds the MAC header to thepackets and send them to the physical layer for transmission.

Among these layers the MAC is in charge of scheduling in which time slotthe message will be sent. The main objective of this scheduling is torandomize in time the transmissions in order to avoid collisions withneighbors' packets.

The MAC layer must not only schedule the data messages coming from theupper layers but also its own packets (acknowledgements, requests andbeacons).

Messages can accept some delay in their transmission, whileacknowledgements must be sent within the timeslot of the reception.These restrictions and the need for time randomization are the base forthe packets scheduling process.

With reference to present priorities for messages, present FIG. 32 is atable that shows an exemplary message priority list in accordance withthe present protocol subject matter. In general, there are two mainkinds of packets the MAC layer must schedule: The ones coming from theupper layers (LPDU) and the ones generated by the MAC layer. The firstcategory can be divided into two types, data and power outagenotification, while the second category includes synchronizationrequests (SYNC RQST) and acknowledgements (SYNC ACK or SYNC NACK), othermessages acknowledgement (ACK or NACK), beacons, beacon requests anddiscovery beacons. The data messages can have different MAC headerdepending of their nature (monocast, broadcast, broadcast ITP . . . )but they will all be treated in the same way from a schedulingstandpoint.

Some messages must be sent in priority; among all these messages,acknowledgements are the most important. A (N)ACK must be sent in thetime slot the reception of the corresponding monocast message occurred;in the same way the SYNC (N)ACK must be sent in the same time slot asthe corresponding SYNC RQST.

The MAC should normally not send more than one message in a given timeslot, except several forced beacons if the hardware can handle it. Inthe rare case an EP would need to acknowledge more than one message orsynchronization requests in the same time slot, then one should be sentand the other cancelled. The reason is that the EP who transmitted theinitial message or request expects an acknowledgement in this time slotand considers the transmission a failure after that (it is thus uselessto transmit a (N)ACK or SYNC (N)ACK after the current time slot).Although packets other than acknowledgements are initially randomized ina window, they are not absolutely restricted to it and can be postponed.

The requests are next in the priority list, with the SYNC RQST justbefore the RQST_Beacon.

All these packets are needed for the network to work properly and arethus higher in priority than the data to transport. The data is next onthis list of priority, followed by the beacons (on a forced channel ornot). These beacons are actually MAC headers used to givesynchronization information. Since the same information is in all MACheaders, if any message is transmitted in the window where an unforcedbeacon is requested, this beacon does not need to be transmitted.Concerning the forced beacons, which are triggered by the reception of adiscovery beacon, they need to be sent in the corresponding listeningwindow, but only if there is a time slot available: including a new nodeto the network must not disturb the nodes already synchronized.

There are two exceptions that supersede the order of priority definedabove: the first one is when an EP experiences a power outage, the APIlayer notifies it to the NET layer. This request changes the normalreceiving mode into a passive, power saving mode interrupted bytransmission of short outage notifications. If another EP receives oneof these notifications, it will re-transmit it with a priority order ofa normal data. The second case is during the discovery phase, where thisorder is meaningless since the MAC only transmits discovery beacons orlistens for “forced” beacons.

Scheduling a message consists in deciding in which time slot it will betransmitted. There are several restrictions that apply to this task.First it must follow the priority rules described in the previoussection; this priority is applied when two messages should be sent inthe same time slot. Additional rules are needed to define thisscheduling task.

As said above the acknowledgements are the highest priority and theyalso must be sent in the same time slot as the message orsynchronization request that triggered them. The acknowledgements cannotbe pushed in time as the messages can (all message can be postponed,excepted the forced beacons which have also a defined window but are ofthe lowest priority and can be cancelled to make place to any otherpacket if needed).

As a result of this first rule, if a message was scheduled in the sametime slot as the reception of monocast data, then this message must bepushed by 1 time slot, to allow the MAC layer to acknowledge thereceived packet. Only acknowledgements can be sent in a time slot wherea LPDU packet was received.

In order to not overload the network any LPDU and SYNC RQST must berandomized over time. The SYNC RQST randomization is done at the MAClayer and is discussed in another section.

Each time a packet is received from the LLC layer, the MAC layer adds itto a FIFO dedicated to data messages. If no data packet is being sentthe MAC layer checks if there is a message in this FIFO. If it is thecase then the transmission window is updated (see Traffic load controlsection) and a countdown timer is randomly determined. This timer isdecremented at each time slot beginning and when it reaches zero themessage is sent during the time slot.

There are several exceptions to this rule. If a higher priority messageis already scheduled or an acknowledgement is expected then the messageis left in the FIFO and the countdown timer set to expire for the nexttimeslot. On the contrary if a forced beacon was scheduled in the sametime slot the data message is scheduled (and/or in the next one for amonocast packet) then the forced beacon is cancelled.

The forced beacon should be sent in the listening window of the endpointin discovery phase. It should be randomized in this window. If the timeslot is already taken then the next time slot should be tested foravailability, cycling to the beginning of the listening window if theend is reached. This procedure should continue until a space is found totransmit the forced beacon or the endpoint realizes that all time slotsare already occupied, in which case it should cancel the forced beacon.

When a packet already scheduled is pushed in time to let the place foran acknowledgement, then all the packets scheduled later are pushed bythe same time slot amount. This should concern only SYNC RQST and BeaconRQST, since the data packets stay in the FIFO until the time slot wherethey are sent (at which point it has already been determined that noacknowledgement was expected).

Finally, concerning there are several rules concerning the transmissioninside a given time slot. The data messages are always sent at thebeginning of the first sub-time slot; this maximizes the space availablefor data and allows the endpoints to send their acknowledgements in thelast sub-time slot.

The synchronization acknowledgements should also be sent in the sametime slot as the request; the SYNC RQST should be sent in the secondsub-time slot and the corresponding acknowledgement in the last sub-timeslot, whether the answer in negative or positive. If the SYNC RQSTtriggers a RQST Beacon to check the connection with one father then itshould also be sent in the last sub-time slot (where the SYNC (N)ACKwould be sent if the connection to a father was good).

The beacons should be randomized between the second and the fifthsub-time slot to not interfere with the start of data messages or theacknowledgements. The same rule should apply to the MAC outage packet.

In other present versions of the protocol the acknowledgments are madein the time slot following the message or the request, which meant thatdata packets could not be sent in successive time slots. The presentversion does not have this restriction, but it is compatible with thefact to not send data back to back if the hardware cannot handle it. Theacknowledgements were put in the same time slot to be in the samefrequency as the original packets and not to gain time. Summarized forthe present version:

-   -   (N)ACK must be sent in the same time slot the reception of a        monocast message occurred.    -   SYNC (N)ACK must be sent in the same time slot the reception of        a SYNC RQST occurred.    -   (N)ACK, SYNC (N)ACK and RQST Beacon are sent in the last        sub-time slot.    -   Beacons are randomized between the 2nd and the 5th sub-time        slot.    -   If a packet is pushed by one time slot then all packets already        scheduled are pushed.

The present discussion more particularly relates to various outagenotification system aspects of the present subject matter. Specifically,it is noted that endpoints that experience a power outage possessimportant information that if it could be relayed to the data collectionsystem, can be applied for very useful network management purposes.However, during a power outage, the supply of energy has been cut off.For low cost devices which do not contain energy storage devices, thismeans that they have limited energy available and will not be able tocontinue to participate to the network. The problem then arises how tomove this valuable information to the cell relay under thesecircumstances.

The present solution is advantageously based on using to relay theinformation the endpoints that do not experience a power outage. At eachpower out, there will be a self-defining fringe where endpoints withinthe power out zone will be able to communicate with endpoints that arestill having power.

Endpoints that experience the power outage will enter a power out modeonce a power failure is detected. The will immediately cease normalnetwork operation and will initiate a few short power out messages overa randomized time window to avoid collisions. As it is still able tokeep time accurately due to oscillator drift compensation, it will beable to select correct frequency channels and time to ensure poweredendpoints in range will be able to capture these messages. Once powerendpoints capture a power outage message, it will be able to store thisinformation and send it using normal network protocol.

If the network connectivity has been influenced by the power outage,endpoints will use the normal network self management functions tore-establish connectivity if possible. Power out information is storedduring this time and is not lost. If the power outage zone is large onlya percentage of power out messages will be reported but should besufficient to infer true outage problems with correlation withelectricity network information, at least from the perspective ofnetwork management purposes.

More particularly, per present subject matter, when a power outageoccurs, the MAC layer enters in a special mode (requested by API). TheMAC layer stops listening and sends 3 very short messages with the EPremaining energy. Each such message is randomized (but still alignedwith the timeslots) in a 5 s window. These outage messages are processedby everyone that can hear them. These messages are also numbered with anoutage number (1 increment by outage, not by sent messages). If beforethe first outage message is sent, the EP recovers its power, then itcancels the outage notifications (but the API layer is free to send apower recovery message). But if the power comes back after the firstoutage message is sent, then the EP will send the remaining two.

When a neighbor that is still powered hears an outage message, its MAClayer indicates to the NET layer (through the LLC) the outagenotification, the neighbor address, the outage number and the time whenthe outage message arrived. It will be the task of the NET layer toforward this information to the Cell Relay in the same way that is usedfor regular (or classical) uplink messages.

The present protocol subject matter beneficially affords analysis ofother aspects of network related performance. Specifically, an embeddedRF environmental evaluation tool is provided to gauge the performanceneeds of RF transceivers. In particular, a statistical radio environmentanalysis tool is embedded in the nodes of a subject mesh network for thepurpose of providing guidelines for the improvement of the hardware.

The present system is intended for use in ISM bands. These bands usuallyfeature a very high level of uncontrolled interference. Thespecifications of the RF hardware, as well as the expected performanceof the network, strongly depend on the electromagnetic environment inthese bands. Two aspects of this environment need to be considered. Thefirst one is the path loss or propagation conditions. Although a lot ofinformation is available on this topic for ISM bands, no reliablestatistical data is available for the specificelectricity-meter-to-electricity-meter situation relevant to thisnetwork. The second aspect of the problem is the interference level. Theknowledge of this parameter is very important because the largest partof the cost of an RF transceiver is associated with interferences andhow to efficiently fight them. The present subject matter provides forthe implementation of an embedded environment analysis tool in theprotocol. This is potentially an invaluable tool for network diagnosisand planning. It will also be the stepping-stone for a next generationhardware definition for any system because it will provide a means tosupport any cost reduction of the RF hardware, by providing extensiveenvironment analysis back up to ensure that any resulting new hardwarehas the required specifications to work in its environment. For suchpurposes, the nodes of the network will probe the electromagneticenvironment with the RSSI (Received Signal Strength Indicator) functionof the receiver. Due to the continuously changing nature of thisenvironment, a very large number of RSSI measurements are necessary tobe valid from a statistical standpoint. Therefore, to avoid clutteringthe limited bandwidth with all these measurements, a statistical dataprocessing will be applied in the node. In this way, only significantinformation will have to be reported to the application. Two differentkinds of environment analysis are specified in this protocol. The firstone is used to explore the time characteristics of the interference andis a measurement of the RSSI auto-correlation function. The second onefocuses on the intensity of the interference and is a measurement of theRSSI probability density function. The first kind of analysis will helpanswering questions like: what is the optimal packet length to avoidcollisions with the other users of the band? The second analysis willhelp answer questions like: what is the necessary adjacent channelrejection to avoid collisions? What is the probability for a collisionto occur if two nodes are some distance away from each other?

A chief benefit is that it enables optimization of RF hardware thatneeds to work in specific conditions, for practice of the presentsubject matter in a particular field environment.

Considering such present environmental analysis tools in greater detail,it will be understood that the specifications of the RF hardware, aswell as the expected performance of the network, strongly depend on theelectromagnetic environment. Two aspects of this environment need to beconsidered. The first one is the path loss or propagation conditions.The second aspect of the problem is the interference level. The MAClayer can probe the electromagnetic environment with the RSSI functionof the receiver, and obtain a relatively large number of RSSImeasurements for statistical validity. Two different kinds ofenvironment analysis are specified in this protocol. The first one isused to explore the time characteristics of the interference and usesthe RSSI auto-correlation function. The second one focuses on theintensity of the interference and uses the RSSI probability densityfunction.

With respect to the subject environmental analysis functionality, theobjective of the RSSI Autocorrelation Acquisition Mode is to measure theaverage RSSI and its auto-correlation function on a single channel. Inthis mode, the endpoint will interrupt for some time its normal hoppingsequence and its usual communication tasks. The MAC layer will configureits receiver in continuous reception mode and will request RSSI readingsfrom the PHY layer at a rate given by the MAC layer parameterRSSI_Sampling_Rate. These readings will then be processed to extract theaverage value and the auto-correlation function. The LLC layer sends therequest to the MAC layer with two input arguments: the channel number toperform the analysis on and a maximum number of samples used toterminate the environment analysis.

The average RSSI, RSSI_avg, will be computed iteratively as explained bythe following algorithm:

Initialization: RSSI_avg = 0 n = 0 For each RSSI reading do$\left\{ {\begin{matrix}\left. n\leftarrow{n + 1} \right. \\{{{RSSI\_ avg}(n)} = {\frac{{RSSI}(n)}{n} + {\frac{n - 1}{n}{RSSI\_ avg}\left( {n - 1} \right)}}}\end{matrix}\quad} \right.$ If n = maximum number of readings, then stopacquisition process end where we have used the following definitions:$\left\{ {\begin{matrix}{{{RSSI}(n)} = {{current}\mspace{14mu}{RSSI}\mspace{14mu}{reading}}} \\{{{RSSI\_ avg}(n)} = {{new}\mspace{14mu}{value}\mspace{14mu}{of}\mspace{14mu}{RSSI\_ avg}}} \\{{{RSSI\_ avg}\left( {n - 1} \right)} = {{old}\mspace{14mu}{value}\mspace{14mu}{of}\mspace{14mu}{RSSI\_ avg}}}\end{matrix}\quad} \right.$

For the computation of the auto-correlation function it is necessarystore in memory the last 100 values of the RSSI. The auto-correlationfunction will only be evaluated for a restricted set of delay values.This set of values is:RSSI_AF_Delays={0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60,70, 80, 90, 100}These values correspond to the time delays

$\frac{{RSSI\_ AF}{\_ Delays}}{{RSSI\_ Sampling}{\_ Rate}}.$

At each RSSI reading, the averaged RSSI is first updated and then theauto-correlation function values are updated. The update process for theRSSI auto-correlation function values, AF_RSSI(m,n), is:

Initialization AF_RSSI(m) =0 for each m ∈ RSSI_AF_Delays After eachRSSI_avg update do For each m ∈ RSSI_AF_Delays do If n ≧ m + 1 temp =(RSSI(n) − RSSI_avg(n))(RSSI(n − m) − RSSI_avg(n))${{AF\_ RSSI}\left( {m,n} \right)} = {\frac{temp}{n - m} + {\frac{n - m - 1}{n - m}{AF\_ RSSI}\left( {m,{n - 1}} \right)}}$end endwhere we have used the following definitions:

$\left\{ {\begin{matrix}{{{RSSI}(n)} = {{current}\mspace{14mu}{RSSI}\mspace{14mu}{reading}}} \\{{{RSSI\_ avg}(n)} = {{new}\mspace{14mu}{value}\mspace{14mu}{of}\mspace{14mu}{RSSI\_ avg}}} \\\begin{matrix}{{{{AF\_ RSSI}\left( {m,n} \right)} = {{new}\mspace{14mu}{value}\mspace{14mu}{of}{\mspace{11mu}\;}{RSSI}}}\mspace{14mu}} \\{{auto}\text{-}{correlation}\mspace{14mu}{function}\mspace{14mu}{for}\mspace{14mu}{delay}\mspace{14mu} m}\end{matrix} \\\begin{matrix}{{{AF\_ RSSI}\left( {m,{n - 1}} \right)} = {{old}\mspace{14mu}{value}\mspace{14mu}{of}{\mspace{11mu}\;}{RSSI}}} \\{{auto}\text{-}{correlation}\mspace{14mu}{function}\mspace{14mu}{for}\mspace{14mu}{delay}\mspace{14mu} m}\end{matrix}\end{matrix}\quad} \right.$

When the requested number of RSSI readings is reached, the acquisitionand update process stops. The RSSI_avg value and the AF_RSSI(m) valuesfor each delay m are then reported to the LLC layer in the confirmationmessage. This report will then be forwarded to Net layer and to the API,which will send it to the Cell Relay. The structure of the outputarguments for the confirmation is shown below:

$\begin{matrix}\begin{matrix}{RSSI\_ avg} & {{AF\_ RSSI}(0)} & {{AF\_ RSSI}(1)}\end{matrix} & \ldots & \begin{matrix}{{AF\_ RSSI}\;(90)} & {{AF\_ RSSI}\;(100)}\end{matrix}\end{matrix}$RSSI_avg is a 1-byte field and the AF_RSSI(m) are 2-byte fields. Afterthis environment analysis, the MAC layer resynchronizes its time slotsand resumes its previous communication activities.

Per the present subject matter, it should also be noted that thisenvironment analysis process must be short enough to prevent theendpoint from loosing its synchronization with the mesh network.Moreover, the auto-correlation acquisition mode should be usedpreferably in nodes not too close from the Cell Relay in order to avoiddisrupting the data flow through the network.

With respect to the subject environmental analysis functionality, theobjective of the RSSI PDF Acquisition Mode is to measure the probabilitydensity function (PDF) of the RSSI readings on three selected channels.In this mode, the endpoint keeps hopping its channel number according tocell hopping sequence, as in normal mode. The node carries on with allits usual communication tasks and uses its spare time to probe theenvironment.

Three different channels are designated for RSSI PDF acquisition andthey are part of the basic hopping sequence. The LLC layer sends therequest to the MAC layer with four input arguments: the three channelnumbers to analyze and a maximum counter value (RSSI_PDF_Max_Count) usedto terminate the environment analysis. The RSSI PDF acquisitionprocedure is described further herein, and present FIG. 33 providesrepresentation illustrations concerning such subject matter.

Whenever the receiver hops to one the channels selected for RSSImeasurement, it will request a RSSI reading of the PHY layer. Only oneRSSI reading is requested per time slot. This RSSI value will then beused to update the RSSI PDF for that channel. The PDF is an array of 24bins, each one of these bins corresponds to a RSSI range of 3 dB asshown in present FIG. 33. A counter is associated with each bin. Forinstance, if the RSSI reading is equal to −113 dBm, the counterassociated with bin 2 will be incremented. In a general way:

Initialize all bin_k_counters to zero RSSI_min = −118 dBm RSSI_step = 3dB If RSSI_min + (k−1)*RSSI_step <= RSSI < RSSI_min + k*RSSI_step thenbin_k_counter = bin_k_counter + 1 if bin_k_counter = RSSI_PDF_Max_Countthen     exit acquisition process end end where bin_k_counter associatedwith bin k.

There are some exceptions to this rule. If the RSSI value is above theupper bound of the last bin, the counter associated will the last binshould be incremented. In a similar way, if the RSSI value is below thelower bound of the first bin, the counter associated with the first binshould be incremented. If a start of frame delimiter is detected in atime slot, prior to the RSSI reading, this time slot should not be usedfor RSSI PDF update. If a start of frame delimiter is detected in a timeslot, after the RSSI reading, this time slot can be used for RSSI PDFupdate. The purpose of this is to prevent Linknet traffic frominterfering with the PDF acquisition. The goal of this measurement is toget an image of the external interferences sources on the channels usedby the network. Each one of the bin counters starts from zero when theprocess is initialized and has a max range of 4095. When any of thesecounters reaches the maximum value RSSI_PDF_Max_Count (≦4095), the RSSIPDF acquisition for that channel is stopped. When the RSSI PDFacquisition is completed for the three selected channels, the MAC layercan report the results to the LLC layer. The structure of the output isshown in present FIG. 34.

If the node loses synchronization, switches to another, cell orexperiences a power outage (it could be done at power up), all thebuffered messages should be deleted.

The MAC layer uses several types of messages to manage its numeroustasks. Not every message contains the same level of information. Inorder to save RF bandwidth and to not send useless bytes, the MAC headerwill be different for almost each message type. However, every messageshould provide synchronization information to anyone who can hear it.This way no message will be useless, even if a message is heard by anendpoint that is not the destination.

At the MAC layer level, the address of an endpoint is the serial numberof the meter itself. Address is so fixed and unique, and is 4 Byteslong.

The message frame at MAC level is composed of:

-   -   A MAC header: It represents all the parameters needed at the MAC        level. We can split it in sub sections: a common part for all        the message types, and a dynamic part.    -   A frame body, which is the LLC Protocol Data Unit, called LPDU.    -   A Frame Check Sequence section, which is the bytes needed to        compute the error detection on the MAC header and the frame        body.

Present FIG. 35 below shows all the fields that can be present at MAClevel. The field and different message structures are otherwisedescribed herein.

According to the present subject matter, there is a part of the headerwhich is common to all types of messages:

LV, Layer Version: It indicates the version of the MAC layer.

FT, Frame Type: These bits indicate the type of the frame. See relativesections of present FIG. 36 for MAC frame type related information. Notethat the message types are arranged in order of priority.

SA, Sender Address: The Sender/Source Address is 4 bytes long.

The MAC header has a dynamic section, in which the fields below don'tappear in every message. They are described here in a general way, withmore details otherwise stated herein for each message type.

RS, Registration State:

-   It indicates whether the endpoint is registered to a cell or not.    This information is provided by the NET layer.-   The RS bit of the Cell Master is always 1.    RSD, Reserved.-   Not used for the moment. This field should be set to 0.    CD, Connectivity Degree-   This field indicates the degree of connectivity of the node with its    fathers. Depending on the number of potential SYNC fathers the node    has, the Connectivity Degree takes a different value. The CD value    of the Cell Master is always set to the maximal value.    CSI, Cell Size Indicator.-   It indicates how full the cell is.    LVL, Level:-   This field indicates the level of the sender. A 0-level transmitter    signals that the transmitter is not connected to the mesh network. A    1-level transmitter signals that the transmitter is a Cell Master.    For the other value, if n is the number of hops to reach the Cell    Master, the level is defined by LVL=n+1.    GPD, Global Propagation Delay:-   This field gives the global propagation delay between the sender and    the Cell Master.    SACT, Slotted Aloha Countdown Timer.-   This field indicates the remaining time before the endpoint switches    to the next time slot when the transmitter sends the message. SACT    is expressed in AMC_SACT_Resolution μs.    TSN, Time Slot Number:-   This field gives the time slot number in which the message is sent.    Combined with the cell address (CELL), any endpoint can deduce the    channel of this time slot.    CELL, Cell address:-   This field gives the address of the cell with which the endpoint is    synchronized. These 2 bytes are used to compute the frequency    hopping sequence used in the cell. In a Discovery Beacon, this field    is used to specify the preferred cell in warm start. The Cell    address is based on the C12.22 address of the Cell Relay WAN board.    FID, Frame ID:-   The Frame ID number is incremented and sent at each Sync Request and    monocast data; it is sent in the (N)ACK or SYNC_(N)ACK to specify    the packet they are answering to.    OID, Outage ID:-   The outage number of the endpoint that experiences a power failure.    DA, Destination Address:-   The Destination Address is 4-byte long.    HFN, HyperFrame Number:-   The hyperframe number can be used in several ways depending on the    message type.    RITP, Relative ITP:-   The relative ITP is propagated in the network through a dedicated    type of message. This is the ITP timestamp of the beginning of the    hyperframe number 0.    RxC, Reception Channel-   This field indicates the channel number on which the EP will listen    during the listening window of the discovery phase    NDB, Number of remaining Discovery Beacons-   It gives the number of remaining discovery beacons to send before    the beginning of the listening window.

The frame body is only present in data message, that is, messages fromthe above layers. This field doesn't exist for the other messages:

LPDU, LLC Protocol Data Unit: This field carries the message for thelayers above the MAC layer.

The Frame Check Sequence (FCS) field is used to detect potential errorsin the frame:

CRC, Cyclic Redundant Code: These 4 bytes are allocated for a CRC-32value to check the integrity of the MAC header and the frame body.

Beacons are empty messages with no specific destination. They onlycontain synchronization information; an endpoint sends beaconsperiodically when it does not generate any other traffic. The beaconlength at MAC level is 19 bytes, as graphically represented by presentFIG. 37.

A SYNC request is sent by an endpoint which wants to become synchronizedwith another. The FID field is a counter, incremented at each sent SYNCRequest or Monocast Data. The SYNC request length at MAC level is 24bytes, as graphically represented by present FIG. 38.

The following types of messages are used to acknowledge or not datamessages and SYNC requests. The FID field refers to the FID of themessage which is (non-)acknowledged. The (N)ACK or SYNC NACK length atMAC level is 24 bytes, as graphically represented by present FIG. 39.

A SYNC ACK message is an acknowledgment of a SYNC request. The FID fieldrefers to the FID of the SYNC request message which is acknowledged. Itdiffers from the SYNC NACK because the current hyperframe number, HFN,and the relative ITP of this hyperframe are also transmitted. The SYNCACK length at MAC level is 29 bytes, as graphically represented bypresent FIG. 40. One special note is that this message doesn't fit in asingle sub time slot (for 1 byte).

If an endpoint needs to update the synchronization information of one ofits neighbors or just checks that it is still present, it can send it arequest beacon. The Request Beacon length at MAC level is 23 bytes, asgraphically represented by present FIG. 41.

The monocast data message is a data message sent to only onedestination. It contains in the frame body the LPDU to carry. The FIDfield is a counter, incremented at each sent Monocast Data or SYNCRequest. The monocast MAC frame length is LPDU length+24 bytes, asgraphically represented by present FIG. 42.

The broadcast is a data message not addressed to anybody in particularbut intended to any endpoint receiving it. The broadcast MAC framelength is LPDU length+19 bytes, as graphically represented by presentFIG. 43.

The ITP messages are dedicated messages, initialized by the Cell Master,to propagate the RITP in the entire cell. They are considered asbroadcast data messages with no frame body. The RITP field is fixed bythe Cell Master when it initializes the message as the HFN field, whichis the hyperframe number of the creation of the RITP. When EPs forwardthe ITP message, they keep the same HFN and RITP fields as the onescreated by the Cell Master. The ITP length at MAC level is 24 bytes, asgraphically represented by present FIG. 44.

The discovery beacon is a short message sent during the discovery phaseby non-synchronized EPs. RxC field indicates the listening channel ofthe discovery phase, NDB the number of remaining discovery beacons tosend before the listening window, and CELL is set to the cell addressthe node wants to synchronize with, in warm start (set to 0 in coldstart). The Discovery Beacon length at MAC level is 13 bytes, asgraphically represented by present FIG. 45.

Outage messages are the simplest and shortest messages that can be sent.When an endpoint realizes that there is power failure, it uses its lastgasp to send several of these messages. The OID gives the outage number.At each new outage, the EP increments this outage number, whichrollovers on 1 byte. For each outage, three outage messages are sentrandomized in three intervals of 5 seconds (the OID stays the same forthese three packets). The Outage message length at MAC level is 10bytes, as graphically represented by present FIG. 46.

Present FIG. 47 schematically represents MAC Layer Services, whichreflect functionality advantageously provided per the present protocolsubject matter. Specifically, with an objective to send a message to onedestination, MAC_Request_Send_Mono_Data, there is use of requisite inputarguments: LPDU, Destination Address. The operation may be described asthe LLC layer requests the MAC layer to send a message to one neighbor.The message is sent with the standard traffic management rules. Notethat the neighbor is not necessarily in the neighbor table.

With an objection to send a broadcast message,MAC_Request_Send_Broadcast, there is use of requisite input arguments:LPDU. The operation may be described as the LLC layer requests the MAClayer to send a message to everybody in the neighborhood. The message issent with the standard traffic management rules.

With an objective to send an RITP timestamp, MAC_Request_Send_ITP, thereis no required use of input arguments. The operation may be described asthe NET layer requests the MAC layer, by the way of the LLC, to send anITP message to everybody in the neighborhood. This request follows thesame approach as a MAC_Request_Send_Broadcast, except that there is noLPDU to carry. Instead of LPDU, the MAC adds in its header the RITP andthe HFN of the creation of this RITP. The message is sent with thestandard traffic management rules.

With an objective to measure the medium interference level on aspecified channel and its time auto-correlation function,MAC_Request_Environment_Analysis_Auto-Correlation, there is use of therequisite input arguments: Channel number, Number of samples. Theoperation may be described as the API layer requests, by the way of theLLC and NET layers, the MAC layer to measure the RSSI on a specifiedchannel, a given number of times. The MAC layer will then compute theaverage value of this RSSI as well as its auto-correlation function. Thesampling acquisition rate of the RSSI is a MAC parameter,MAC_RSSI_Sampling_Rate. The values of the delays for the computation ofthe auto-correlation function are given by another MAC parameter,MAC_RSSI_AF_Delays.

With an objective to measure the medium interference level on threespecified channels and its probability density function,MAC_Request_Environment_Analysis_PDF, there is use of the requisiteinput arguments: Channel numbers (3), RSSI_PDF_Max_Count (maximum valueof bin counters). The operation may be described as the API layerrequests, by the way of the LLC and NET layers, the MAC layer to measurethe RSSI on three specified channels taken from the hopping sequence.For each one of these channels, the MAC layer will compute the RSSIprobability density function (PDF). The acquisition process stops when abin counter reaches the maximum allowed value for that request.

With an objective to give to the MAC layer the information whether acell is authorized or not, MAC_Request_Cell_Authorization, there is useof the requisite input arguments: Cell Address, Cell Status. Theoperation may be described as the NET layer gives, by the way of the LLClayer, to the MAC layer the status of a cell. This one can beauthorized, or forbidden.

With an objective to give to the MAC layer the information whether theNET layer is registered, MAC_Request_Cell_Registration, there is use ofthe requisite input arguments: Cell Address, Registration Status. Theoperation may be described as the NET layer informs, by the way of theLLC layer, of the NET registration status in the cell. Then the MAClayer can put the RS (Registered State) bit to 0 or 1 in its header.

With an objective to answer a MAC_Request_Send_Mono_Data,MAC_Confirmation_Send_Mono_Data, there is use of the requisite outputarguments: Status of the message (ACK, NACK, No ACK). The operation maybe described as it gives to the LLC layer the status of a Send Mono DataRequest. Each confirmation is linked to the number of the associatedrequest to avoid confusion. The confirmation can be a ACK or NACK ifsuch messages have been received, or a No ACK status if the destinationhas not answered in the time slot it should had.

With an objective to answer a MAC_Request_Send_Broadcast andMAC_Request_Send_ITP, MAC_Confirmation_Send_Broadcast andMAC_Confirmation_Send_ITP, there is use of the requisite outputarguments: Status. The operation may be described as it confirms to theLLC when the broadcast or the ITP has been sent, Then the LLC canproceed to the repetition if necessary.

With an objective to answer aMAC_Request_Environment_Analysis_Auto-Correlation,MAC_Confirmation_Environment_Analysis_Auto-Correlation, there is the useof requisite output arguments: average RSSI, RSSI auto-correlationfunction values. The operation may be described as the MAC layer sendsto the layer above the RSSI average value and the values of theauto-correlation function computed during the requested environmentanalysis. The number of values for the auto-correlation function is thenumber of delays at witch this function was calculated. These delays aredefined by the MAC layer parameter MAC_RSSI_AF_Delays.

With an objective to answer a MAC_Request_Environment_Analysis_PDF,MAC_Confirmation_Environment_Analysis_PDF, there is use of the requisiteoutput arguments: RSSI PDF values for the three requested channels (3×24values). The operation may be described as the MAC layer sends to thelayer above the RSSI PDF values (actually the values of the bincounters) for each one of the three requested channels.

With an objective to answer a MAC_Request_Cell_Autorization,MAC_Confirmation_Cell_Authorization, there is the use of the requisiteoutput arguments: Status. The operation may be described as it justconfirms that the request has been taken into account.

With an objective to answer a MAC_Request_Cell_Registration,MAC_Confirmation_Cell_Registration, there is use of the requisite outputarguments: Status. The operation may be described as it just confirmsthat the request has been taken into account.

With an objective to forward a received LPDU message to the LLC layer,MAC_Indication_Received, there is use of the requisite output arguments:LPDU, Sender Address. The operation may be described as it gives to theLLC layer the LPDU that has been received with the sender address.

With an objective to inform that a broadcast ITP message has beenreceived, MAC_Indication_ITP_Received, there is no use of any outputarguments. The operation may be described as when a valid broadcast ITPmessage is received, the MAC layer updates the RITP field and informsthe NET layer, by the way of the LLC layer, of that arrival. The resultof this indication forces the NET layer to forward this ITP if notalready done.

With an objective to update the ITP of the API layer,MAC_Indication_ITP_Update, there is the use of the requisite outputarguments: Absolute ITP Timestamp. The operation may be described as theRITP can be updated after the reception of an ITP broadcast message or aSYNC ACK. The MAC layer then computes an update of the absolute ITP andsends it to the API layer. As soon as the ITP is computed, it should begiven to the API layer very quickly. This indication has priority overall other indications.

With an objective to indicate to the above layer the MAC state,MAC_Indication_State, there is the use of the requisite outputarguments: State, Cell address. The operation may be described as theMAC layer informs the upper layers after each state modification. TheMAC layer can be not-synchronized or synchronized with a cell. In thelast case, the address of the cell is indicated.

With an objective to indicate to the above layer that the MAC layer willsoon leave the current cell, MAC_Indication_Cell_Leaving process, thereis no use of any output arguments. The operation may be described as theMAC layer informs the upper layers that it has found a new cell and itwill leave soon the current one.

With an objective to inform that a power outage notification has beenreceived, MAC_Indication_Outage_Received, there is use of the requisiteoutput arguments: Outage ID, Sender Address, Outage Time. The operationmay be described as it indicates to the NET layer, by the way of theLLC, that a neighbor has experienced an outage at a given time. Itforces the Net layer to forward this notification to the Cell Master.

The MAC layer is organized in three modes: Non-Synchronized Mode,Synchronization Mode, and Synchronized Mode. When the meter is switchedon or after a Black Out, the MAC layer goes into Non-Synchronized Mode.Present FIG. 48 represents such features and other aspects, includingadditional disclosure regarding MAC State Diagram subject matter.

The Logical Link Control layer is the second sub-layer of the Data Linklayer, next to the Net layer. Its goal is to provide otherfunctionalities for the Data Link layer, without burdening the MAClayer. These functionalities can independently operate above the MAClayer ones but still have the goal to optimize access to the medium.

Some services provided by the MAC layer are useless to the LLC layer,they are rather intended for layers above the Data Link layer, such asthe NET layer. Therefore, in order not to break the layer approach, someservices are merely forwarded from the MAC to the NET layer, and viceversa, going through the LLC layer as if it was a pipe.

The following listing describes the adjustable (that is, tweakable)parameters of the LLC layer with their associated preferred defaultvalues being referenced in present FIG. 49.

LLC_Duplication_Table_Size

-   -   Description: The number of LLC message entries saved in memory        to avoid duplicate messages when the same packet is received        several times.

LLC_Max_Message_Length

-   -   Description: The maximum message length that the LLC layer        allows the upper layer to send.    -   Comment: this includes the service of fragmentation and thus is        directly obtained by the equation:        LLC_Max_Message_Length=LLC_Max_Packet_Length*LLC_Max_Nb_of_Packets

LLC_Max_Nb_of_Packets

-   -   Description: The maximum number of packets the LLC can fragment        a message into.

LLC_Max_Nb_of_Downlink_Transmissions:

-   -   Description: The maximum number of times a downlink packet        should be sent if no acknowledgement is received.

LLC_Max_Nb_of_Uplink_Transmissions:

-   -   Description: The maximum number of times an uplink packet should        be sent if no acknowledgement is received.

LLC_Max_Packet_Length

-   -   Description: The maximum NETPDU packet length that the LLC layer        allows the upper layer to send in one time slot.    -   Comment:        LLC_Max_Packet_Length=MAC_Max_Packet_Length−LLC_Header_Length

LLC_Message_Timeout

-   -   Description: Specifies a time limit for the        transmission/reception of a fragmented message.

LLC_Nb_of_Broacast_Transmissions

-   -   Description: The number of times a broadcast is sent.

LLC_Tx_Retry_Exp_Offset

-   -   Description: This parameter controls the initial slope of the        exponential backoff law for repetition.

LLC_Tx_Retry_Exp_Range

-   -   Description: This is used to compute the repetition number from        which the binary exponential backoff law is truncated.

LLC_Tx_Retry_Exp_Start

-   -   Description: The repetition number from which the binary        exponential backoff law is applied for transmission retry        randomization window computation.    -   Comment: This value must be smaller than the maximum number of        transmissions for uplink and downlink packets for the        exponential backoff to be used.    -   Comment: This value must be smaller than        (LLC_Max_Nb_of_Transmissions−LLC_Tx_Retry_Exp_Start) for the        truncation to take place.

Each monocast message sent by the LLC layer should be acknowledged atthe MAC layer level. It will happen very often than the MAC layer willreport that it didn't receive this acknowledgement. This can occur ifthe destination endpoint fails to receive the message or if the senderfails to receive the acknowledgement, due to collisions, interference orfading. In any case of failure, the LLC layer will send the messageagain until it is acknowledged or until the maximum number of attemptsis reached. After LLC_Max_Nb_of_Downlink_Transmissions, orLLC_Max_Nb_of_Uplink_Transmissions, unsuccessful transmissions, the LLClayer reports a No-ACK failure status to the NET layer.

For broadcast data or ITP, the LLC will automatically repeat the messageusing the backoff algorithm once the MAC layer has notified that theformer sending is gone and this until the specified number of attempts,Number_of_Broacast_Transmissions, is reached.

When the LLC layer receives from the NET layer a request to send apacket, or when it reschedules a non-acknowledged transmission, it willintroduce a random delay before actually sending the request to the MAClayer. The purpose of this delay is to avoid flooding the air interfacewith a large number of packets when the conditions of transmission aredifficult. The LLC layer will compute the length of a randomizationwindow, Tx_Wait, and will send the actual request to the MAC layer aftera random delay with a uniform probability distribution between zero andTx_Wait. The value of Tx_Wait is computed as a function of therepetition number. Tx_Wait is computed according to a truncated binaryexponential backoff law as given by the following equation:

${Tx\_ Wait} = \left\{ \begin{matrix}0 & {{{if}\mspace{14mu} R} \leq R_{start}} \\{2^{R - R_{start}} - 1} & {{{if}\mspace{14mu} R_{start}} < R < {R_{start} + R_{range}}} \\{2^{R_{range}} - 1} & {{{if}\mspace{14mu} R} \geq {R_{start} + R_{range}}}\end{matrix} \right.$

Here R is the repetition number: it ranges from zero for the firsttransmission attempt to some maximal value given by(LLC_Max_Nb_of_Transmissions−1). The application of this exponentialbackoff law is delayed and truncated as can be seen from the aboveequation. This is also illustrated by present FIG. 50, which graphicallyrepresents delayed truncated binary exponential backoff for transmissionretries if packets are not acknowledged. The rationale behind this issimple and can be explained in the following way. In the “no wait time”period, the transmitter is trying different channels to avoidinterference or difficult propagation conditions. In the “binaryexponential backoff” period, the transmitter is progressively increasingthe wait time to allow the network to recover from unusually difficultconditions (whatever they are). The “truncated exponential backoff” isnecessary to avoid introducing unrealistically long wait times in thesystem.

The beginning and end of the binary exponential backoff law are given bytwo LLC layer parameters:

R_(start)@LLC_Tx_Retry_Exp_Start

R_(range)@LLC_Tx_Retry_Exp_Range

An additional parameter introduces an offset in the application of theexponential law and gives a way to control the initial slope:

-   R_(offset)@LLC_Tx_Retry_Exp_Offset

The following example illustrates the retransmission randomizationwindow algorithm for R_(start)=5, R_(range)=5, R_(offset)=2,LLC_Max_Nb_of_Transmissions=15, and TS_Length=1. The first column (R=0)corresponds to the initial transmission attempt.

R 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Tx_Wait 0 0 0 0 0 0 4 12 28 60 124124 124 124 124

If a packet is negatively acknowledged (NACK), the “no wait time” phaseof the retransmission strategy should be bypassed because in this casewe know that what happened is not due to a propagation problem orinterference (see appendix). For similar reasons, in the special case ofbroadcast transmission, the non delayed exponential backoff should beused. Broadcasts generate a burst of traffic and internal collisionsbetween nodes of the network are likely to occur and slow down thebroadcast. The immediate use of an exponential backoff can mitigate thisproblem. See also present FIG. 51, which graphically representstruncated binary exponential backoff for transmission retries if packetsare negatively acknowledged. In all these cases the repetition law isgiven by:

${Tx\_ Wait} = {{TS\_ Length} \times \left\{ \begin{matrix}{2^{R_{offset}}\left( {2^{R} - 1} \right)} & {{{if}\mspace{14mu} R} < R_{range}} \\{2^{R_{offset}}\left( {2^{R_{range}} - 1} \right)} & {{{if}\mspace{14mu} R} \geq R_{range}}\end{matrix} \right.}$

If a packet is not acknowledged and later negatively acknowledged on asubsequent transmission attempt, the retry law for negativelyacknowledged packets should be applied. In the same way, if a packet isnegatively acknowledged and later a retry is not acknowledged, the retrylaw for negatively acknowledged packets should be kept.

The LLC layer offers a non-duplication service. Because RF mediagenerate a large number of collisions, the LLC layer can send a messagemore than once, and also receive the same packet several times. To avoidforwarding the received message to the NET layer several times, eachtransmitted packet has a LLC number, LLC FID. See present FIG. 52, foran example of an LLC Duplication Table. Due to this number, the LLClayer knows whether the packet has already been received. Should thishappen, the packet is discarded.

At each new reception, the message number, the packet number and thesender address are saved in a FIFO, which contains properties of theLLC_Duplication_Table_Size last messages. So the oldest entry isreplaced by the new one if not already present in the table. If aduplicated message is received, the existing associated entry in thetable needs to be removed and rewritten as a new entry. This will ensurethat this entry remains longer in the table.

It should be noted that at the reception of a message, the detection ofduplicated packets should be done before the operation of reconstitutionof a fragmented message.

If the node loses synchronization, switches to another cell, orexperiences a power outage (it could be done at power up), theduplication table should be cleared.

Another service offered by the LLC layer is the fragmentation ofmessages. The LLC layer offers the upper layer to send messages oflength up to LLC_Max_Message_Length bytes, a size that can be longerthan the one offered by the MAC layer, which is limited by the time slotlength. LLC_Max_Message_Length is limited by the physical size of thedevice memory that runs the protocol and by the fact the LLC layercannot handle more than 15 packets.

If the NET layer asks to send a message larger than what the MAC layercan send, the LLC layer splits the message into several shorter packets.The packet number and the number of packets are mentioned in the LLCheader to enable the reconstitution of the entire message at reception.

Each packet corresponds to an individual request sent to the MAC layer.The MAC layer treats these packets as regular complete messages.

The LLC layer computes the required number of packets depending on themessage length and the maximum size the MAC layer can handle.

From a transmitter side perspective, the LLC layer splits the messageinto packets. A MAC request is associated with each packet. When thefirst packet is sent, a timeout counter of LLC_Message_Timeout length isstarted. Each packet can be sent several times, with the same repetitionlimitation as for a standard packet, until the packet is acknowledged bythe MAC layer. When all the packets have been acknowledged, the LLClayer confirms to the NET layer that the message has been sent withsuccess. If one packet has not been sent correctly or if the counterreaches LLC_Message_Timeout, the LLC layer informs the NET layer thatthe transmission has failed.

From the receiver side perspective, the receiver LLC layer when itreceives the first packet of a fragmented message, starts the samecounter of LLC_Message_Timeout length as that of the transmitter side.When all the packets have been received, the LLC layer regenerates theentire message and gives it to the NET layer. If the counter reachesLLC_Message_Timeout value and at least one packet is still missing, allthe other packets are deleted.

LLC_Message_Timeout value should be long enough to let the MAC layersend correctly all the packets. The value can depend on the number ofpacket to send.

The present protocol subject matter offers a kind of broadcast service,or more specifically, a multicast one. The function is that the samemessage can be sent to all the endpoints of a cell. This is a truebroadcast, which is not acknowledged but simply transmitted Number_ofBroacast_Transmissions times by each endpoint. It is intended to anyendpoint hearing it (except the Cell Master, where the broadcastsoriginate).

The LLC header is 3 bytes long, as represented by present FIG. 53, whichotherwise represents the full LLC Frame. Within such representation, thefollowing additional frame information is applicable, as will beunderstood by one of ordinary skill in the art without further detailedexplanation.

LV, Layer Version:

-   -   The layer version number

RSD, Reserved:

-   -   Not initially used. Set to 0.

FID, Frame ID:

-   -   This byte indicates the message number to avoid duplication of a        packet.

NOP, Number Of Packets:

-   -   These 4 bits give the number of packets that have to be        transmitted to rebuild the data. It implies that the protocol        can split messages in a maximum of 15 packets.

PN, Packet Number:

-   -   These 4 bits give the packet number of the fragmented data. Zero        corresponds to the first packet.

Additionally, the NET Protocol Data Unit (NETPDU) contains informationrelative to the network layer.

The LLC Layer has a variety of interfaces and associated services(functionality), as represented in detail per present FIG. 54. The LLClayer ensures reliable functionality, not only of itself but in theservices it provides for those in relationship with it, such asresulting in the LLC layer managing repetition and fragmentation ofmessages. For example, depending on (non-) acknowledgements, the LLClayer chooses if the packet has to be retransmitted or not.

With an objective to send a message to one destination,LLC_Request_Send_Mono_Data, there is use of requisite input arguments:NETPDU, and Destination Address. The operation may be described as theNET layer requests the LLC layer to send a message to one of itsneighbors. The LLC layer fragments the message into several packets ifneeded before giving it to the MAC layer. The LLC layer also tries tosend the message several times before reporting a success or an error tothe NET layer.

With an objective to send a message to the neighborhood,LLC_Request_Send_Broadcast, there is use of requisite input argumentsNETPDU. The operation may be described as the NET layer requests the LLClayer to send a message to all the endpoint's neighbors. The LLC layerfragments the message into several packets if needed before giving it tothe MAC layer. The broadcast is repeatedNumber_of_Broacast_Transmissions times.

With an objective to send an RITP timestamp to the neighborhood,LLC_Request_Send_ITP, there is no use of input arguments. The operationmay be described as the NET layer requests the LLC layer to send an ITPmessage to all the endpoint's neighbors. This request follows the sameapproach as a LLC_Request_Send_Broadcast, except that there is neitherNETPDU, nor LPDU. The LLC forwards the request to the MAC layer, whichis in charge of time stamping the message. The LLC layer manages therepetition of this message as a regular broadcast.

With an objective to measure the medium interference level on aspecified channel and its time auto-correlation function,LLC_Request_Environment_Analysis_Auto-Correlation, there is use ofrequisite input arguments: Channel number, Number of samples. Theoperation may be described as a request being forwarded directly to theMAC layer.

With an objective to measure the medium interference level on threespecified channels and its probability density function,LLC_Request_Environment_Analysis_PDF, there is use of requisite inputarguments: Channel numbers (3), Max value of counter. The operation maybe described as being a request forwarded directly to the MAC layer.

With an objective to give to the MAC layer the information whether acell is authorized or not, LLC_Request_Cell_Authorization, there is useof requisite input arguments: Cell Address, Cell Status. The operationmay be described as a request being forwarded directly to the MAC layer.

With an objective to give to the MAC layer the information whether theNET layer is registered, LLC_Request_Cell_Registration, there is use ofrequisite input arguments: Cell Address, Registration Status. Theoperation may be described as a request being forwarded directly to theMAC layer.

With an objective to answer an LLC_Request_Send_Mono_Data,LLC_Confirmation_Send_Mono_Data, there is use of requisite outputarguments: ACK, NACK, No ACK, Destination Address of the sent packet.The operation may be described as it confirms to the NET layer whetherthe message has been successfully sent to destination or not. If not andif at least a NACK has been received, it should be notified to the NETLayer. The NET layer is then able to choose whether it has to transmitthe packet to another destination. Upon receiving a failure confirmationfor an uplink message, the NET layer will update its routingprobabilities and send a new request to the LLC.

With an objective to answer an LLC_Request_Send_Broadcast andLLC_Request_Send_ITP, LLC_Confirmation_Send_Broadcast andLLC_Confirmation_Send_ITP, there is use of requisite output arguments:OK. The operation may be described as confirms to the NET layer that thebroadcast has been sent.

With an objective to answer anLLC_Request_Environment_Analysis_Auto-Correlation,LLC_Confirmation_Environment_Analysis_Auto-Correlation, there is use ofrequisite output arguments: average RSSI, RSSI auto-correlation functionvalues. The operation may be described as a forward ofMAC_Confirmation_Environment_Analysis_Auto-Correlation from the MAClayer to the NET layer.

With an objective to answer an LLC_Request_Environment_Analysis_PDF,LLC_Confirmation_Environment_Analysis_PDF, there is use of requisiteoutput arguments RSSI PDF values for the three requested channels (3×24values). The operation may be described as a forward of aMAC_Confirmation_Environment_Analysis_PDF from the MAC layer to the NETlayer.

With an objective to answer an LLC_Request_Cell_Autorization,LLC_Confirmation_Cell_Authorization, there is the use of requisiteoutput arguments: Status. The operation may be described as a forward ofMAC_Confirmation_Cell_Authorization from the MAC layer to the NET layer.

With an objective to answer an LLC_Request_Cell_Registration,LLC_Confirmation_Cell_Registration, there is use of requisite outputarguments: Status. The operation may be described as a forward ofMAC_Confirmation_Cell_Registration from the MAC layer to the NET layer.

With an objective to forward a received NETPDU message to the NET layer,LLC_Indication_Received, there is use of requisite output arguments:NETPDU, Sender Address. The operation may be described as afterassembling all the packets if the message was fragmented, the LLC layergives the received NETPDU to the NET layer.

With an objective to inform that a broadcast ITP message has beenreceived, LLC_Indication_ITP_Received, there is no use of any outputarguments. The operation may be described as a direct forward of theMAC_Indication_ITP_Received from the MAC layer to the NET layer.

With an objective to update the ITP of the API layer,LLC_Indication_ITP_Update, there is use of the requisite outputarguments: Absolute ITP Timestamp. The operation may be described as adirect forward of the MAC_Indication_ITP_Update from the MAC layer tothe NET layer. This indication has priority over all other indications.

With an objective to indicate to the above layer the MAC state,LLC_Indication_State, there is the use of requisite output argument:State, Cell address. The operation may be described as the LLC layergets this indication directly from the MAC_Indication_State.

With an objective to indicate to the above layer that the MAC layer willsoon leave the current cell, LLC_Indication_Cell_Leaving_process, thereis no use of any output arguments. The operation may be described as adirect forward of the MAC_Indication_Cell_Leaving_process from the MAClayer to the NET layer. Before forwarding, the LLC layer frees itsbuffers and pending actions.

With an objective to inform that a power outage notification has beenreceived, LLC_Indication_Outage_Received, there is the use of requisiteoutput argument: Outage ID, Sender Address, Outage Time. The operationmay be described as a direct forward of theMAC_Indication_Outage_Received from the MAC layer to the NET layer.

The following relates more particularly to the network (NET) layer. Thenetwork layer is the third layer of the OSI model and the highest layerof the Linknet protocol. It is the heart of the routing mechanism. Allendpoints have the same network layer except the Cell Master, which hasextended routing functions. The NET layer main task is to decide whatthe destination of messages is, but it is also in charge of the cellregistration process, which is internal to the RF LAN.

Per the EP (End Point) NET layer feature, the NET layer forwards anymessage to the next hop. It also provides data on its neighborhood tothe cell relay. Per the CR (Cell Relay or Cell Master) NET layer, theNet layer offers to send a message to a particular EP (downlink message)in the cell or to the entire cell (broadcast message). The CR NET layerdoesn't offer to send a message to a specific set of EPs in the cell.Also, the network layer is active only while the endpoint issynchronized, and it will let the application layer use the network onlyif it is registered at the NET level.

NET Parameters may be listed as follows, including their respectiveidentifications, descriptions, and default values:

NET_Broadcast_Lift_Expectancy:

-   -   Description: the maximum lifetime of a broadcast in a cell, in        number of hyperframes.

NET_CR_Downlink_Duplication_Table_Size:

-   -   Description: The number of NET Downlink message entries saved in        the Cell Master memory to avoid duplicate downlink messages when        received several Broken Link messages from the same Downlink.

NET_CR_Duplication_Table_Size:

-   -   Description: The number of NET message entries saved in the Cell        Master memory to avoid duplicate messages when the same packet        is received several times.

NET_Downlink_Life_Expectancy:

-   -   Description: the maximum lifetime of a downlink in a cell, in        number of hyperframes.

NET_Endpoint_TimeOut.

-   -   Description: The time after which an endpoint should be removed        from the Cell Master routing table if no message has been        received from it.

NET_EP Duplication_Table_Size:

-   -   Description: The number of NET message entries saved in the        endpoint memory to avoid duplicate messages when the same packet        is received several times.

NET_Max_Registration_Attempts:

-   -   Description: The maximum number of attempts of Cell Registration        Request an endpoint should send to a given cell before declaring        this cell as forbidden.

NET_Max_Nb_of_EPs:

-   -   Description: The maximum number of endpoints by cell. This is        useful for the Cell Master NET layer, which can decide to        authorize or not a new endpoint in its cell. It is also the size        of the Cell Master routing table.

NET_Nb_of_Fathers_Routing:

-   -   Description: When routing a message to the Cell Master, the NET        layer will chose a father among a subset made up of the best        fathers for routing. The size of this subset is        NET_Nb_of_Fathers_Routing.

NET_Nb_of_Endpoints_Neighbor List:

-   -   Description: It corresponds to the maximum number of endpoints        present in a Neighbor List.

NET_Neighbor_List_First_Time:

-   -   Description: The time after which an endpoint should send its        first Neighbor List to the Cell Master when it becomes        synchronized with a cell, in a warm start case.

NET_Neighbor_List_Max_Period:

-   -   Description: The maximum period between two transmissions of the        Neighbor List to the Cell Master, if this list didn't change.

NET_Neighbor_List_Min_Period:

-   -   Description: The minimum period between two transmissions of the        Neighbor List to the Cell Master, if this list changed.

NET_Reg_Send_Config_Period:

-   -   Description: The rate at which the Cell Master should try to        send the Registration Confirmations that are pending. It can be        slower if the downlink buffer is full but it should not be        faster.

NET_Registration_Retry:

-   -   Description: The period during which an endpoint is waiting for        the Registration Confirmation to its first request. After the        first request this time is multiplied by the number of request        tries to get the wait period.

NET_Registration_Send_Max:

-   -   Description: this parameter defines the upper limit of the NET        randomization window in which the Registration Request should be        sent.

NET_Registration_Send_Min:

-   -   Description: this parameter defines the lower limit of the NET        randomization window in which the Registration Request should be        sent.

NET_Uplink_Lift_Expectancy:

-   -   Description: the maximum lifetime of an uplink in a cell, in        number of hyperframes.

With respect to the so-called Neighbor table, the network layer uses theNeighbor table of the MAC layer, with reading rights. That means thatthe network layer can't modify values in such table. Present FIG. 55depicts NET LAYER PARAMETER DEFAULT VALUES subject matter.

Before authorizing upper layers to use the network, an endpoint shouldbe registered at a NET layer level. NET registration process starts assoon as the endpoint gets its synchronization from the MAC layer. Thereare two ways to proceed whether the endpoint was previously registeredwith the cell, which leads to a warm start process, or if it is joininga new cell, which leads to a cold start process.

The behavior during this cell registration process can also be viewedfrom two sides, the endpoint or the Cell Master.

In the Cold Start scenario, the following events that must happen beforethe API layer can access the Network:

-   -   The MAC layer gets synchronization with one of its potential        SYNC fathers and enters in a new cell.    -   The NET layer is informed of the synchronization status and        sends, randomly between NET Registration_Send_Min and        NET_Registration_Send_Max, a Cell Registration request to the        Cell Master.    -   Then the endpoint is waiting for the answer:    -   1. If there is no answer. There is a retry mechanism every        (number of retries*NET_Registration_Retry). There is a maximum        of NET_Max_Registration_Attempts registration attempts with this        cell before giving up; after that the cell is declared as        forbidden and the MAC layer should look for another cell (it is        reminded that after MAC_Unsynchronized_Timeout days in discovery        mode, the MAC layer will re-authorize all the cells).        -   So with NET_Registration_Send_Min=5 min,            NET_Registration_Send_Max=10 min, NET_Registration_Retry=1            hour, and NET_Max_Registration_Attempts=5, the requests will            be done within the following windows:        -   1^(st) try: 5-10 min, 2^(nd): 65-70 min, 3^(rd): 185-190            min, 4^(th): 365-370 min, 5^(th): 605-610 min.        -   The timeout corresponds to the window start of the next try            (or to NET_Max_Registration_Attempts*NET_Registration_Retry            for the last try; 15 hours)    -   2. If a NET Cell Out notification is received, the cell is        immediately marked as forbidden and the MAC layer will looked        for another cell.    -   3. If a NET Cell Registration confirmation is received, the        endpoint switches to NET Registered state.        -   Once the endpoint is NET Registered:    -   1. The NET informs the MAC layer that will now set its RS bit to        1 in its header, which means that this endpoint can now be used        as a SYNC father for its neighbors.    -   2. The NET informs the API layer that it is now authorized to        send messages through the network.    -   3. The NET layer starts sending periodically neighbor lists (the        Cell Registration request is considered as one, so the first        real neighbor list is sent after one defined period; i.e.        NET_Neighbor_List_Min_Period hour if no change occurred).

Several particular items of note are:

-   -   At any moment, if an endpoint receives a NET Cell Out        notification, it should leave the cell    -   While the endpoint is not registered, it cannot be used as SYNC        father, as it doesn't meet potential SYNC father condition        (RS=0)    -   During the Registration confirmation waiting time, no neighbor        list is sent.    -   As the API layer is not authorized to talk while the endpoint is        not yet registered, the uplink with neighbor list packet type is        never used. However, this packet definition is still defined in        the spec., as it may be used one day.

In the Warm Start condition circumstances, it is much simpler andfaster:

-   -   The MAC layer gets synchronization with one of its potential        SYNC fathers and recovers the cell in which it was previously        registered.    -   The NET layer directly goes in registered state and informs the        MAC and API layers.    -   The NET layer starts sending periodically neighbor lists. Since        no cell registration request, which contains neighbor list        information, was sent, the first Neighbor List is sent        Neighbor_List_First_Time minutes after.

When a Cell Master receives a Cell Registration requests it should senda Cell Registration confirmation and update its routing table with thenew information (add the endpoint if it is not already there) unless therouting table is full, in which case it should send a Cell OutNotification. For more detail on the actions of a Cell Master whenreceiving a packet, see the remaining disclosure herewith.

During a cold start, the Cell Master is receiving a lot of CellRegistration Requests in a narrow time window. It has therefore notenough time to send all the Registration confirmation at the same rateit receives requests. As a consequence, the Cell Master should keeptrack of the requests it receives and then sends the confirmation whenit has time to do it. During a cell cold start, the number of “pendingregistrations” can be very high (around half of cell size).

The pending registration list can be handled by setting a flag in therouting table the corresponding endpoints (1 bit*max cell size=1 kb=125B) and periodically sweep the table to know which endpoints needs aregistration confirmation.

The pending registration could also be done as a list of addresses tosend confirmation to; this is a tradeoff between memory space and codeexecution time and the choice is let to the implementation.

In both cases, when there are many stacked confirmations, they should besent at a maximum rate of NET_Reg_Config_Send_Period. It is to ensurethat we are not ourselves flooding the network with downlink messages.

When an endpoint switches cell, it will register in the new cell. Theissue is that it is still registered in the old cell; this is not anissue for routing since the API level will realize that the endpointmoved cell but this can be an issue for cell size management.Effectively the cell size indicator (CSI) used in the cell switchingprocess is based on the number of endpoint in the NET routing table ofthe Cell Master and if this number does not reflect the reality then theswitching process does no longer work properly. This is why it isimportant that the old Cell Master be informed of the departure ofendpoints from its cell. This is the role of the Cell LeavingNotification message.

As soon as the MAC layer notifies that a neighbor from a better cell isreachable, it plans a SYNC request and waits for the SYNC ACK. The SYNCACK received, the MAC layer triggers some actions at each layer:

-   -   MAC:    -   Save timing information relative to the new cell and neighbor.    -   Start a Cell Switching timer of        MAC_CELL_Leaving_Process_Duration seconds.    -   LLC:    -   Free Buffers and pending actions    -   NET:    -   Free Buffers and pending actions    -   Send a Cell Leaving Notification to the current Cell Master.        This message will allow the cell master to remove the Endpoint        from its routing table and update the cell size.

During MAC_CELL_Leaving_Process_Duration seconds, the endpoint acts asusual except it cannot decide to start the same process with a thirdcell. It is implied that during this period the endpoint will haveenough time to send the Net message AND also to forward the same kind ofinformation from sons which decide themselves to change cell (theyprobably see the same new cell at the same time).

When the timer expires, whatever the status of remaining messages tosend, the MAC layer resynchronizes its clock and switches to the newcell. LLC and NET layer are notified by this switching. Therefore, theyfree again their buffers, pending actions . . . The NET layer (and theAPI) is then able to register with the new Cell Master.

The entire routing algorithm is based on the fact that every endpoint inthe cell knows its 1-hop neighborhood and knows nothing beyond that1-hop range. The situation is slightly different for the Cell Master andwill be explained in a dedicated section.

The protocol features two different routing directions, uplink anddownlink. Uplink is used by a meter to send a message to the Cell Masterand downlink is used to convey a message from the Cell Master to ameter. These two routing directions use different mechanisms asexplained below.

Another routing mechanism is the broadcast, used to convey a message toevery endpoint of the cell.

Each packet that is going through the NET layer contains a timestamp ofwhen it was generated. Each time a packet is routed at the NET layerlevel, the lifetime of the packet is checked against its life expectancyand it is deleted if is too old.

More particularly, to perform an uplink communication, the endpoint hasto send its message to the Cell Master. The endpoint does not know whereits Cell Master is but it knows that its fathers can reach it.Therefore, it has to send the uplink message to one of its fathers. Theprotocol should limit its choice to the NET_Nb_of_Fathers_Routing bestfathers, based on the SYNC_Merit. It should randomly select one of thesefathers with a probability for each father inversely proportional to itsSYNC_Merit, and then send the uplink message to that father. Everysynchronized and registered father of the cell is suitable for uplinkrouting.

If the LLC reports a failed transmission (after all the retries), theNET layer will look again in the neighbor table (which is now updatedaccording to the results of the previous transmission attempts) andreselect a random father among the NET_Nb_of_Fathers_Routing best ones,using the same probabilistic selection algorithm as in the firstinstance. The uplink message is then forwarded to the LLC layer fortransmission to the newly selected father. This process continues untilthe LLC reports a successful transmission or until the endpoint goes topower fail, becomes unsynchronized or switches to a new cell.

When the selected father receives this uplink message, it will proceedin a similar fashion. This process is repeated until the best father isthe Cell Master.

Before an Uplink packet is routed to one of the endpoint's fathers, theNET layer check that it is not older than NET_Uplink_Life_Expectancy. Ifit is older then it is deleted.

In contrast, for a downlink path, that is, sending a message from theCell Master to a specific endpoint, is a very easy thing as far asendpoints are concerned. Indeed the entire path is indicated in thenetwork header. This path is specified by the Cell Master, which has acomplete knowledge of all the endpoints in the cell.

When an endpoint receives a downlink message, it reads its networkheader and automatically finds the next destination to contact. This isrepeated until the final destination is reached. Before forwarding themessage to the next node, the endpoint removes its own address in therouting path of the NET header. The endpoint should forward the messageto next address in the header, even if this address doesn't match anynode in its own neighbor table.

Before a downlink packet is forwarded to the next hop, the NET layercheck that it is not older than NET_Downlink_Life_Expectancy. If it isolder then it is deleted.

If, for any reason, the next node cannot be reached (after the LLC layerretries), the message has to be sent back to the Cell Master by theuplink path. This message is called a Broken Link message. A Broken Linkmessage is made of:

-   -   The original downlink message    -   The intended final destination address    -   The NET frame ID of the downlink message    -   A new NET frame ID for the Broken Link    -   The Addresses of the missing link.

It will be the task of the Cell Master to find another path, by takinginto account network changes that occurred in the mid-time. The CellMaster will then re-send the downlink message if a path to thedestination is still available.

There is no acknowledgement at network layer level. If the user of theprotocol wants to be sure that its downlink message has reached thefinal destination, it has to request it in the application layer.

As noted later the Cell Registration Confirmation and the Cell OutNotification are special cases of downlink packets which do not generatea broken link.

A broadcast message is only initiated at the Cell Master level. All theendpoints connected to the cell should receive the broadcast message. Inorder to simplify this heavy operation in a large cell, the protocoldoes not guarantee that the message will be delivered to 100% ofendpoints and does not provide for an acknowledgement at the networklayer level. Upper layers should manage repetition of the message tothose that haven't received it. This can be done by addressing themessage to each remaining endpoint by a common downlink path. Thenetwork layer doesn't either inform upper layers of which endpointshaven't been reached (if it was the case, the network layer wouldproceed to the repetition itself).

The broadcast technique is based on the fact that each endpoint repeatsthe broadcast message Number_of_Broacast_Transmissions number of timesand each endpoint can receive the broadcast from any other endpoint. Forone broadcast message sent by the Cell Master, an endpoint will receiveas many replicas as it has neighbors; this mechanism allows a goodcoverage of the cell. Note that the Cell Master should never accept abroadcast message, as it always generate them.

A filter mechanism is implemented at the network layer level to sendonly once the broadcast message to the application layer and also toforward it once. A broadcast message number (NET FID) is contained inthe NET header to remove messages that have already been received (samemechanism as in the LLC layer). With this duplication detection thebroadcast will not be repeated infinitely in the cell.

The duplication detection will work as long as there are less broadcastsbeing forwarded in the cell than there are spaces in the table ofduplication detection. This should normally be the case if the networkis used properly. As a failsafe, a timeout mechanism has been added. Atinitialization of the broadcast, the Cell Master accesses the currentHyperframe Number (HFN) and inserts it into the NET header. When anendpoint in the cell receives this broadcast it should always accept it(after checking if it is not a duplication), but only retransmit it ifthe broadcast is not too old by comparing the HFN contained into theheader and the current HFN. The maximum lifetime of a broadcast in thecell is NET_Broadcast_Life_Expectancy.

The NET layer offers a non-duplication service for:

-   -   Uplink path        -   Uplink (with or without Neighbor List)        -   Neighbor List        -   Broken Link        -   Outage Notification    -   Broadcast

Broadcast and Outage Notifications can be relayed using several paths inthe network. This redundancy contributes to a better quality of messagedelivery. But it can also increase drastically the amount of traffic.For other messages using uplink routing mechanism, duplication alsohappens but less often. Notice that the downlink path is not concernedhere as it cannot be duplicated. Using this duplication detectionfeature with Downlink messages will even degrade performances of theservice.

To avoid forwarding the received message several times to the API layeror to the next endpoint, each transmitted message has a NETidentification number, NET FID or NET OID. Due to this number, the NETlayer knows whether the packet has already been received. Should thishappen, the packet is discarded.

At each reception of a message, the message number (NET FID or OID) andthe sender address are written in a dedicated table called the NETduplication table (see, for example, present FIG. 56). This tablecontains the properties of the NET_EP_Duplication_Table_Size lastmessages and is managed as a FIFO buffer. If the table is full when anew message comes in, the oldest entry of the table is thrown out as thenew one is entered into the table. If the new entry is identical toanother entry in the table, this indicates that a duplicate message hasbeen received. In this case the oldest copy is removed from the table asthe new one is entered into the table. In this way the information willremain longer in the table.

The Cell Master duplication table size can be extended toNET_CR_Duplication_Table_Size. It will ensure a better filtering beforeforwarding messages to the API layer, mainly useful for OutageNotifications that can be very numerous in a short period of time. For aBroadcast (coming from the Cell Master), the sender address field shouldbe set to zero. For an Outage Notification, the sender address shouldcorrespond to the original sender address specified in the ORG field. Inthe particular case of a Broken Link, the sender address is specified inthe BRKS field. If the node loses synchronization, switches to anothercell or experiences a power outage (it could be done at power up), theduplication table should be cleared.

With reference to a Neighbor list, if the knowledge of the 1-hopneighborhood is enough for an endpoint, it is not the case for the CellMaster. This one has to know the content of the cell to compute downlinkpaths. Therefore, all the endpoints should send regularly their NETNeighbor List using an uplink message.

The Neighbor List is generated from the NET neighbor table. It consistsof the NET_Nb_of Endpoints_Neighbor_List best fathers, i.e. fathers withlowest SYNC_Merit, represented by their MAC addresses. Endpointaddresses are sorted, the best one appears first in the list. Thisinformation is enough for the Cell Master to route packets to anyspecific endpoint.

As an option, if the number of fathers present in the neighbor table isinferior to NET_Nb_of Endpoints_Neighbor_List, the Neighbor List can becompleted with the best brothers (if they are valid for routingpackets). However the downlink path algorithm should be clever enough todetect circular path.

Only synchronized and registered neighbors of the cell can be members ofthe Neighbor List.

The first Neighbor List is sent NET_Neighbor_List_First_Time after theendpoint becomes synchronized to a cell for a warm start, andNET_Neighbor_List_Min_Period for a cold start.

Neighbor Lists should be updated every time an endpoint is removed fromthe list or a new endpoint is added to the list. Neighbor Lists are thensent at a period of NET_Neighbor_List_Min_Period if a change occurred inthe previous period. If an endpoints changes level this should raise aflag that will trigger the transmission of a Neighbor List at the end ofthe current period. If an endpoint undergoes a cell switch, then itshould be in cold start in its new cell and should apply thecorresponding process of registration.

If such a change doesn't occur, Neighbor Lists are sent with a period ofNET_Neighbor_List_Max_Period.

A randomization time of ±20% should be added to these periods and timesto avoid repetitive collisions.

The Cell Master is the only endpoint that can initialize a Broadcastmessage and in the same way the only one that can't receive one. Itshould indicate in the NET header the current hyperframe number (HFN) atthe creation time.

Part of the advantageous features of the present protocol subject matterrelate to a downlink routing mechanism. In particular, the downlinkcommunication represents a transmission from the head of the network toa single node in the mesh network. However, the point is to find a nodethat could be everywhere but which one must be able to access it in theshortest time.

The following more particularly addresses cell relay functionality inrelation to a routing table. To perform a downlink communication, theCell Master (or relay) needs to have received first a minimal number ofNeighbor Lists in order to compute the best path to reach thedestination.

As every endpoint in the cell is supposed to have sent information oftheir best fathers, the Cell Master has the knowledge to reach anyendpoint in the cell. It just has to arrange the information it receivesinside a routing table. This routing table is updated each time a newNeighbor List (or Broken Link message; see dedicated section) isreceived.

Because each endpoint knows its fathers, the Cell Master can constructall the possible paths between any endpoint and itself (it is in factlimited by the number of fathers in the Neighbor List but this providesenough possible paths without overloading the Cell Master with too muchdata to collect and process). The fathers in the Neighbor lists areprovided in order starting from the best one. The best path can bedetermined by choosing the first father in the list starting from thefinal destination up to the Cell Master.

The table will permanently move, therefore the algorithm needs to checkthat each hop of the creation, the path does not go through the samenode. It could else lead to infinite loop or circular path. If the pathreached such case, the algorithm should go reversely in the path andtries with the alternative fathers. If no path is found, the NET layerdestroys the packet and reports a failure to the API layer. If there isan issue due to a broken link or circular path, this technique does notguarantee that the new path found will be the second best but it ensuresthat it will find a path if there is one.

Each time the Cell Master receives an updated Neighbor List, it will useit to update its routing table. It should be noticed that the CellMaster does only need to replace the corresponding entry in the tablewithout re-computing anything.

When the Cell Master needs to send a downlink message to an endpoint, itselects the best path and mentions all the steps (node addresses) of thepath in the network header. Then it sends the message to the firstendpoint of the path, and this one will forward it to the second, and soon, until the final destination is reached. Every node present in therouting table can be selected for routing.

Finally if no message from an endpoint has been received forNET_Endpoint_TimeOut, the endpoint should be deleted from the table aswell as the paths through it. The API layer can also request from theNET layer to remove a specified node from the table.

Present FIG. 57A is an example of a downlink path computation to reachendpoint 10. The 3 best fathers are mentioned in the Neighbor Lists.Some endpoint that doesn't have 3 fathers put their brothers asalternative paths. Each endpoint's neighbors are here sorted by theirSYNC_Merit. The generated path is: 10←8←7←2←CM.

A downlink message follows the path indicated by the Cell Master, butdue to the possible delay between the routing table update and thisdownlink packet, the network configuration can have and some endpointsmay not be reachable anymore (power failure, cell switching, obstacles .. . ). When the LLC layer reports a failure to transmit the message tothe next destination, the network layer should send back to the CellMaster a Broken Link. If, due to hardware limitation, there is no spaceinto the memory to allocate the broken link in the up-going messagebuffer, this one should be destroyed.

When the Broken Link arrives at the Cell Master, this one should updateits routing table by removing the link. The Downlink is then re-sent ifa path still exists. This Broken Link mechanism does not apply to theCell Registration confirmation or to the Cell Out Notification; fordetails see the cell size management section herein.

Present FIG. 57B is a CR Routing Table for a Downlink routing example.When the number of entries in the routing table changes bracket, theCell Master updates its Cell Size Indicator according to the per presentFIG. 57C and provides this information to the MAC layer. The MAC layerwill indicate this information in its header and propagate it throughthe cell. When the cell is full, i.e. when the CSI takes its maximumvalue; no endpoint can request to join the cell.

When the routing table reaches NET_Max_Nb_of_EPs entries, the table andthe cell are considered full. In this case every new Neighbor List thatarrives at the Cell Master should be used to find a path to send a NETCell Out Notification to this node, and then deleted. The node thatreceives this NET Cell Out Notification should acknowledge the packet atMAC layer and leave the cell after a couple of time slots (to make surethe acknowledgment was successfully received).

The NET Cell Out Notification is a particular case of a downlink messagewith no payload. It should be treated as a downlink message except thatno Broken Link message should be sent if a link is broken on the path.This is justified by the fact that the Cell Master has destroyed allinformation relative to this endpoint and therefore will not be able tofind an alternative path. Note that the NET Cell Out Notification canalso be used by upper management software to suppress a selectedendpoint in a cell. The NET Cell Registration Confirmation should notalso trigger a broken link if the message fails to be delivered. Thereason is that the Cell Registration process has its own retry mechanismwhich handles such failures. When the Cell Master receives a CellLeaving Notification it should remove the corresponding endpoint fromits routing table and update its Cell Size Indicator.

The table of present FIG. 57D summarizes actions in the Cell Master NETlayer when the routing table is full or when a node is not in therouting table.

The following relates more particularly to outage notification transportand to the functional area of cell registration in accordance withpresent subject matter. As it is additionally explained in relation tothe MAC layer section, an EP can hear, at MAC level, that a neighborexperiences a power outage. When this event occurs, the MAC layerindicates it to the Net layer, with the outage ID and the time when themessage has been received as parameters. As a consequence, the Net layershould build a Net outage message, with all these parameters (Neighboraddress, Outage ID, and Outage Time), and send it to the Cell relay as anormal uplink message. When the NET outage message reaches the CellRelay, the message is given to the API layer.

When an endpoint is not NET registered, the API layer can't send anypacket. The RFLAN should inform the API layer when the endpoint becomesregistered or deregistered (this last case happens when the endpointgoes back to discovery phase or switches cell). Note that the CellMaster is always considered NET registered.

Once the Endpoint is NET registered, the API layer is authorized tocommunicate and it can start its own registration process.

When an endpoint switches to another cell, the NET and the API layerswill need to register with this new cell. The NET layer informs firstthe API layer that it is no longer registered to a cell and cannot sendpackets. The process following is similar to the connection to a cellfrom the discovery phase.

The Linknet protocol will be implemented in an architecture that willhave limited storage capacity. There is thus a non negligibleprobability that the allocated memory to save messages prior to be sentcan be full. Each time the API layer asks to send a message, the NETlayer confirms or refuses the request. It means that the NET layer candestroy the packet to send and report a failure to the API layer. TheAPI layer should then be in charge of postponing its request and keepthe packet into its own memory.

The Cell Master NET layer can also report a failure to the API layer ifthe path to the destination can't be computed or if the destination isnot in the routing table.

With respect to the NET Frame, it should be understood that the networkmessage is divided into two parts:

-   -   The network header contains all the needed information to rout        the network message from the source to the destination. It is        divided in 2 parts, respective common and dynamic sections.    -   The network body contains the message to transfer to the API        layer.

Present FIG. 58 illustrates all the fields that can be present at NETlevel. The field and different message structures are otherwisedescribed herein.

In the Common Section of the Network Header, there are the followingaspects, with present FIG. 59 providing a Table which presents variousfacets regarding the NET Frame type information:

LV, Layer Version:

-   -   The version of the network layer

FT, Frame Type:

-   -   The network frame type

In the Dynamic Section of the Network Header, there are the followingaspects, some of which fields don't appear in every message (hence, theDynamic nature of this Header Section). They are described here ingeneral terms, with other details of various of the message typesotherwise discussed herein.

ORG, Original Sender:

-   -   The address of the original sender of the message.

FID, Frame ID:

-   -   The network message number given by the original sender.

PL, Path Length:

-   -   The number of addresses in the Path field

PATH:

-   -   Addresses of the next destinations for the message.    -   For a downlink, the entire path is mentioned. At each hop, the        addressee removes its own address from the path.

RSD, Reserved:

-   -   Not used for the moment. This field should be set to 0.

NBN, Number of Neighbors:

-   -   Number of neighbors of the endpoint

NA, Neighbor Address:

-   -   Address of the neighbor relative to the mentioned properties.

BRK S, BRoKen link Sender:

-   -   The Sender Address of the Broken Link transmission

BRK D, BRoKen link Destination:

-   -   The Destination Address of the Broken Link transmission

HFN, HyperFrame Number:

-   -   HFN field refers to the MAC hyperframe number when the Cell        Master initializes the broadcast.

DW FD, Final Destination:

-   -   The final destination the non-received Downlink message was        intended for. It concerns Broken Link message.

DW FID, DoWnlink Frame ID;

-   -   The NET frame ID of the non-received Downlink message. It        concerns Broken Link message.

OID, Outage ID:

-   -   The Outage number of the endpoint which has experienced a power        failure.

OT, Outage Time:

-   -   The time at which the endpoint experiences the power outage. It        is an ITP format.

With further reference to the Frame Body, and related Application PDU(APIPDU), it contains information relative to the application layer. Itis the final message the protocol delivers to application layer. It canbe delivered to the destination endpoint or to the cell relay.

The following more particularly relates to NET messages. The NET headerlength for an uplink message is 8 bytes, as represented by present FIG.60. The Trace packet is used for debugging purpose. However, the NETheader length for a downlink path is 5 bytes plus 4 bytes times the pathlength, as represented by present FIG. 61. This does not include thenext destination which is passed in parameter to be placed in the MACheader. The ORG field is not included since all downlink messagesoriginate in the cell relay. Additionally, per present FIG. 61, HOP-N isthe address of the final destination of the downlink message, whileHOP-1 is the address of the next destination of the downlink message.

When an endpoint wants to send its Neighbor List to the Cell Master, itinserts it at the location of the frame body. The Cell RegistrationRequest uses the same format but with a different Frame Type. Asrepresented by present FIG. 62, the Neighbor List or Cell RegistrationRequest length at NET level is 21 bytes.

It is a combination of an uplink message and a Neighbor List message. Asrepresented by present FIG. 63, the NET header length for an uplink withNeighbor List message is 21 bytes.

As represented by present FIG. 64, the broadcast NET header length is 4bytes. HFN field refers to the MAC hyperframe number when the CellMaster initializes the broadcast.

When the Cell Master wants to reject one node from the cell, it sends aCell Out Notification to this node. This message length is 5 bytes plus4 bytes times the path length. As represented by present FIG. 65, theCell registration Confirmation uses the same packet with a differentframe type to confirm to a node that it is accepted in the Cell.

If during a downlink transmission an endpoint does not manage to forwardthe message to the next endpoint then a Broken Link is reported. TheBroken Link message consists in the addresses of the 2 EPs defining theBroken Link and the original message (which was not stored in the CellMaster). This Broken Link message will be used by the Cell Master toupdate its routing table and compute a new best path. As represented bypresent FIG. 66, the NET header length of the Broken Link message is 18bytes.

If an EP hears an outage message (at a MAC level) from an EP whichexperiences a power outage, it timestamps the outage notification withits local time using ITP format. It should then create a (NET) outagemessage and send it to the Cell Master in the same way as an uplinkmessage. The OID is the same as in the original MAC outage message. Andthe OT field is the ITP timestamp of the reception of the (MAC) outagenotification. As represented by present FIG. 67, the NET header lengthof the outage message is 11 bytes.

The Cell Leaving Notification is sent by an endpoint just before leavinga cell. This informs the Cell Master that this endpoint can be removedfrom the routing table and the CSI adjusted accordingly. As representedby present FIG. 67, the Cell Leaving Notification NET length is 8 bytes.

With reference to NET interfaces and services, it will be appreciatedthat the Network layer proposes a variety of different services, asillustrated in significant detail by the subject matter included inpresent FIG. 69. The network layer is in charge of routing. The networklayer knows the 1-hop neighborhood and can make a decision as to thepath to take to forward a packet in the Uplink or Downlink direction. Ifthe message has arrived at its destination, either in an endpoint or acell relay (Cell Master), it gives it to the API layer.

The following more particularly relates to functionality of the presentprotocol subject matter with reference to NET Requests.

With an objective to send a message to one destinations,NET_Request_Send_Mono_Data, there is use of input arguments: APIPDU,destination address. The operation may be described as the API layerasks the NET layer to send a message to the Cell Master. For the CellMaster: The API layer asks the NET layer to send a message to oneparticular endpoint.

With an objective to send a broadcast message to the entire cell, as faras Cell Relay only, NET_Request_Send_Broadcast, there is use of therequisite input arguments: APIPDU. The operation may be described as theAPI layer requests the NET layer to send a message to the entire cell.

With an objective to send an RITP broadcast to the entire cell, as faras Cell Relay only, NET_Request_Send_ITP, there is no use of any inputarguments. The operation may be described that the API layer requeststhe NET layer to send an ITP message to the entire cell. This requestfollows the same approach as a NET_Request_Send_Broadcast, except thatthere is no NETPDU. The MAC layer will be in charge of time stamping themessage.

With an objective to update the RITP of the MAC layer, as far as theCell Relay (Cell Master) only, NET_Request_Update_ITP, there is use ofrequisite input argument: Absolute ITP time stamp. The operation may bedescribed as the API layer requests the MAC layer to update its RITPwith a new value of ITP. This request has priority over all otherrequests and is forwarded directly to the LLC layer.

With an objective to measure the medium interference level on aspecified channel and its time auto-correlation function,NET_Request_Environment_Analysis_Auto-Correlation, there is use of inputargument: Channel number, Number of samples. The operation may bedescribed as a request that is forwarded directly to the LLC layer.

With an objective to measure the medium interference level on threespecified channels and its probability density function,NET_Request_Environment_Analysis_PDF, there is use of requisite inputargument: Channel numbers (3), Max value of counter. The operation maybe described as a request that is forwarded directly to the LLC layer.

With an objective to give to the MAC layer the information whether acell is authorized or not, NET_Request_Cell_Authorization, there is useof requisite input argument: Cell Address and Cell Status. The operationmay be described as a request that is forwarded directly to the LLClayer.

With an objective to give to remove a node from the routing table, asfar as the Cell Relay (Cell Master) only, NET_Request_Remove_Node, thereis use of requisite input argument: Node Address. The operation may bedescribed as the API layer can inform the Linknet layers that a node isnot belonging to the cell anymore. The NET layer will so remove the nodefrom the routing table and update the number of endpoint in the cell.

With an objective to send a Cell Out notification to node, as far as theCell Relay (Cell Master) only, NET_Request_Eject_Node, there is use ofrequisite input argument: Node Address. The operation may be describedas a request that the API layer can request the Net layer to get a nodeout of the cell. The NET layer will then send a Cell Out Notification tothis node and remove it from the routing table.

The following more particularly relates to functionality of the presentprotocol subject matter with reference to NET Confirmations.

With an objective to answer a NET_Request_Send_Mono_Data,NET_Request_Send_Broadcast and NET_Request_Send_ITP,NET_Confirmation_Send_Mono_Data, NET_Confirmation_Send_Broadcast andNET_Confirmation_Send_ITP, there is use of requisite output argument:Status. The operation may be described as giving the status of therequest. It can be Message transmitted, Message failed to betransmitted, Buffer Full, Path to destination not found or any otherkinds of errors.

With an objective to answer NET_Request_Update_ITP, as far as Cell Relayonly, NET_Confimation_Update_ITP, there is use of requisite outputargument: Status. The operation may be described as it gives the statusof the request.

With an objective to answerNET_Request_Environment_Analysis_Auto-Correlation,NET_Confirmation_Environment_Analysis_Auto-Correlation, there is use ofrequisite output arguments: average RSSI and RSSI auto-correlationfunction values. The operation may be described as a forward ofLLC_Confirmation_Environment_Analysis_Auto-Correlation from the LLClayer to the API layer.

With an objective to answer NET_Request_Environment_Analysis_PDF,NET_Confirmation_Environment_Analysis_PDF, there is the use of requisiteoutput arguments: RSSI PDF values for the three requested channels (3×24values). The operation may be described as a forward ofLLC_Confirmation_Environment_Analysis_PDF from the LLC layer to the APIlayer.

With an objective to answer NET_Request_Cell_Autorization,NET_Confirmation_Cell_Authorization, there is use of requisite outputargument: Status. The operation may be described as a forward ofLLC_Confirmation_Cell_Authorization from the LLC layer to the API layer.

With an objective to answer a NET_Request_Remove_Node, as far as CellRelay (Cell Master) only, NET_Confirmation_Remove_Node, there is use ofrequisite output argument: Status. The operation may be described as itgives the status of the request.

With an objective to answer a NET_Eject_Remove_Node, as far as CellRelay (Cell Master) only, NET_Confirmation_Eject_Node, there is use ofrequisite output argument: Status. The operation may be described as itgives the status of the request.

The following more particularly relates to functionality of the presentprotocol subject matter with reference to NET Indications.

With an objective to indicate to the API layer that a message wasreceived for it and provide this message, NET_Indication_Received, thereis use of output arguments: APIPDU. The operation may be described aswhen a message reaches its final destination, the NET layer gives themessage to the API layer.

With an objective to update the ITP of the API layer, as far as the CellRelay only, NET_Indication_ITP_Update, there is use of output argument:Absolute ITP Timestamp. The operation may be described as a directforward of the LLC_Indication_ITP_Update from the LLC layer to the APIlayer. This indication has priority over all other indications.

With an objective to indicate to the above layer the NET state,NET_Indication_State, there is use of requisite output argument: Stateand Cell address. The operation may be described as the NET layerindicates to API layer whether the endpoint is synchronized andregistered with a cell. Once it is, the API layer gets the rights to usethe network.

With an objective to indicate to the API layer that a outagenotification has been received, as far as Cell Relay only,NET_Indication_Outage_Received, there is use of requisite outputarguments: EP address, Outage ID and Outage Time. The operation may bedescribed as when a Cell relay (or Cell Master) receives an outagenotification through the RF LAN, it should give it to the API layer. TheAPI layer will report to the collection engine using C12.22 format.

The following relates to stability analysis of the crystal driftcompensation algorithm and indicates how to compute the filtercoefficients. To discuss the behavior of the drift correction process,present FIG. 70 provides a diagrammatical model of the feedback controlloop.

With more specific reference to such FIG. 70, TS_Length_Cell is thedefault time slot length of the cell as seen from the endpoint (that is,as measured with the endpoint's own time reference). Any discrepancybetween the endpoint time reference and the cell time reference willresult in a difference between TS_Length and TS_Length_Cell. Consideringthe loop reaction to a sudden change in TS_Length_Cell, one can adjustthe filter coefficients to make this response well-behaved.

For such analytical purposes, present FIG. 71 is a simplification of thediagram of present FIG. 70. For such simplification, the representedloop may be described by the following recurrence equations:X ₂(n)=AX ₁(n)+BX ₂(n−1)T _(slot)(n)=T _(slot)(n−1)+X ₂(n)X ₁(n)=TS_Length_Cell−T _(slot)(n)

As is well understood by those of ordinary skill in the art, thez-transforms of these equations are:

X ₂(z)=AX ₁(z)+Bz ⁻¹ X ₂(z)T _(slot)(z)=z ⁻¹ T _(slot)(z)+X ₂(z)X ₁(z)=TS_Length_Cell−T _(slot)(z)Solving for X₂ and then for T_(slot):

${X_{2}\;(z)} = \frac{{AX}_{1}(z)}{1 - {Bz}^{- 1}}$${T_{slot}(z)} = {\frac{X_{2}\;(z)}{1 - z^{- 1}} = {\frac{{AX}_{1}(z)}{\left( {1 - z^{- 1}} \right)\left( {1 - {Bz}^{- 1}} \right)} = \frac{A \times {TS\_ Length}{\_ Cell}}{{\left( {1 - z^{- 1}} \right)\left( {1 - {Bz}^{- 1}} \right)} + A}}}$The  final  expression  for  T_(slot)  is:${T_{slot}(z)} = \frac{A \times {TS\_ Length}{\_ Cell}}{1 + A - {\left( {1 + B} \right)z^{- 1}} + {Bz}^{- 2}}$And the transfer function of the loop is:

$\frac{T_{slot}(z)}{{TS\_ Length}{\_ Cell}} = \frac{\frac{A}{1 + A}}{1 - {\left( \frac{1 + B}{1 + A} \right)z^{- 1}} + {\left( \frac{B}{1 + A} \right)z^{- 2}}}$

This is a second order low-pass transfer function. To look at thedynamic behavior, one finds the poles of this function, written here inthe standard form:

$\frac{1}{1 + {b_{1}z^{- 1}} + {b_{2}z^{- 2}}}$${{{where}{\mspace{11mu}\;}b_{1}} = {- \left( \frac{1 + B}{1 + A} \right)}},{b_{2} = \left( \frac{B}{1 + A} \right)}$

The zeros of

${z^{2} + {b_{1}z} + {b_{2}\mspace{14mu}{are}\mspace{14mu} z_{1,2}}} = {{- \frac{b_{1}}{2}} \pm {\frac{j}{2}{\sqrt{{4b_{2}} - b_{1}^{2}}.}}}$Simple relations between b₁, b₂ and the poles of the transfer functionare represented by:

${{{Re}\left( z_{1,2} \right)} = {- \frac{b_{1}}{2}}},{{z_{1,2}}^{2} = b_{2}}$

A simple time-saving approach to evaluate the filter coefficients is tomap the poles of the discrete-time transfer function to the poles of thecorresponding continuous-time transfer function. These poles are givenby s_(1,2)=−ζω₀±jω₀√{square root over (1−ζ²)}, where ζ is the dampingfactor and ω₀ is the angular frequency of the equivalent continuous-timesystem. These poles are related to the poles in the z-plane by Z=e^(sT),leading to the following expression for the poles in the z-plane:

$z_{1,2} = {{\mathbb{e}}^{{- {\zeta\omega}_{0}}T}{\mathbb{e}}^{{\pm {j\omega}_{0}}T\sqrt{1 - \zeta^{2}}}}$

Thereafter, one may write the transfer function coefficients in terms ofthe continuous-time system parameters:b ₁=−2e ^(−ζω) ⁰ ^(T) cos(ω₀ T√{square root over (1−ζ²)}) and b ₂ =e^(−2ζω) ⁰ ^(T).

Solving for A and B gives:

$\quad\left\{ \begin{matrix}{A = {\frac{{\mathbb{e}}^{2\zeta\;\omega_{0}T}}{{2{\mathbb{e}}^{\zeta\;\omega_{0}T}{\cos\left( {\omega_{0}T\sqrt{1 - \zeta^{2}}} \right)}} - 1} - 1}} \\{B = \frac{1}{{2{\mathbb{e}}^{\zeta\;\omega_{0}T}{\cos\left( {\omega_{0}T\sqrt{1 - \zeta^{2}}} \right)}} - 1}}\end{matrix} \right.$

The relevant design parameter is the number of samples used to computethe average (actually it is a low-pass filter and not an average). Thisis arbitrarily defined as the number of samples needed to reach theovershoot of the step response:

$N_{avg} = {\frac{{oscillation}{\mspace{11mu}\;}{period}}{2T} = \frac{\pi}{T\;\omega_{0}\sqrt{1 - \zeta^{2}}}}$

The following equations are obtained to compute the filter coefficientsin terms of N_(avg) and the damping factor ζ:

$\quad\left\{ \begin{matrix}{A = {\frac{\exp\left( \frac{2\;\pi\;\zeta}{N_{avg}\sqrt{1 - \zeta^{2}}} \right)}{{2{\exp\left( \frac{\pi\;\zeta}{N_{avg}\sqrt{1 - \zeta^{2}}} \right)}{\cos\left( \frac{\pi}{N_{avg}} \right)}} - 1} - 1}} \\{B = \frac{1}{{2{\exp\left( \frac{\pi\;\zeta}{N_{avg}\sqrt{1 - \zeta^{2}}} \right)}{\cos\left( \frac{\pi}{N_{avg}} \right)}} - 1}}\end{matrix} \right.$

For example, N_(avg)=16.5 and ζ=0.7 give A= 1/16 and B=0.732.

Present FIG. 72 illustrates the step response of such loop in reactionto a change in the cell time slot length from 100 ms to 110 ms. Thesystem reaches a stable value after about 20 resynchronizations with theexpected overshoot of 4.6%. The following default values for the MAClayer parameters may be used for one preferred exemplary embodiment:

${{MAC\_ Xdrift}{\_ Filter}{\_ A}} = \frac{1}{16}$MAC_Xdrift_Filter_B = 0.732

The use of minimal propagation delay path to optimize a mesh network isanother present advantageous feature set. In a mesh network, a method ispresently proposed to compute to propagation delay between any node ofthe network and the root node of the network. This propagation delay isthen used as criterion to select the best path among several in thenetwork.

The stability and performance of the mesh network is based on itsability to self-optimize and to self-heal its topology. Thisself-optimization of the network is also fundamental to balance andlimit the traffic load. It is therefore important to provide theprotocol with a good path selection algorithm. The best path should havethe following properties:

-   -   The best path should have the shortest latency from transmitter        to receiver.    -   The best path should minimize the number of retransmissions.    -   The best path should cause as few interference as possible.

The present subject matter is to use the “shortest path” criterion tochoose the best path among several. The shortest path is here defined asthe path with the shortest average propagation delay. Reference is madeherein to the diagrammatical illustrations of present FIGS. 73A, 73B,and 73C.

When a communication attempt from node A to node B fails, node Aretransmits the message a second time or as many times as necessary tomake the transmission successful. For practical purposes the number ofretransmission attempts is limited to some maximum value in order toavoid losing time on a broken link. The average time spent to transmit amessage from node A to node B is called in this protocol the localpropagation delay (LPD) from A to B. It is straightforward to extendthis concept to a more complex path. For instance the propagation delaybetween nodes A and D along the path ABCD (see present FIG. 73A) is thesum of the propagation delays from A to B, B to C and C to D.

In order to avoid using too much memory in a node, each node is onlyaware of its immediate neighbors and therefore cannot directly make thesum of all the propagation delays all the way to the root node. To solvethis problem each node in the network will compute two kinds of delays:the local propagation delay (LPD) and the global propagation delay(GPD). A node will compute the LPD for each one if its neighboring nodesin the direction of the network root. The GPD of a node is defined asthe shortest total propagation delay from the node all the way to theroot node of the network. Here shortest means that if there are severalpossible paths from a node to the root node, the one with the shortestpropagation delay is selected and used to define the node's globalpropagation delay (GPD).

As an example, for the network illustrated in present FIG. 73B, the GPDfrom A to the root will be:GPD(A)=Min{[LPD(A,B)+LPD(B,D)],[LPD(A,C)+LPD(C,D)]}

To make the GPD computation possible with only the knowledge of theimmediate neighborhood, the GPD information is propagated step by stepfrom the root to the nodes.

As an example, for the network illustrated in present FIG. 73C, the GPDfrom A to the root will be computed in this way:GPD(A)=Min{[LPD(A,B)+GPD(B)],[LPD(A,C)+GPD(C)]},

where GPD(A) is the total propagation delay from A to the root andGPD(B) is the total propagation delay from B to the root.

The detailed operations to be performed inside a node are then:

-   -   Each node keeps a record of all communication attempts with each        one of the nodes in the direction of the root of the network and        computes a statistical communication success rate with each one        of these nodes.    -   From this communication success rate an average local        propagation delay is computed for each one-hop link.    -   From the GPD of the neighboring nodes, the node will compute the        total propagation delay along each path to the root node and        will select the shortest value to define its own GPD value.    -   The node will then make its GPD value accessible to every node        within its range by updating its message header.

Mathematical details of such present subject matter may be outlined asfollows. If we consider a single hop (point-to-point) link, in acollision free environment the average propagation delay D will be givenby:D=T_(d),where T_(d) is the time needed by a packet to travel from thetransmitter to the receiver.

Considering the effect of collisions on the average propagation delaymeans considering all the possible cases: no collision occurred, onecollision occurred, two collisions, and so on. The propagation delaywill be given by:D=T _(d) P+(T _(d) +T _(r))(1−P)P+(T _(d)+2T _(r))(1−P)² P+ . . . .Here P is the packet success rate and T_(r) is the wait time betweenretransmissions.

To keep the further derivation simple, one may assume that this time isconstant. The expression can be factored in this way:D=T _(d) P[1+(1−P)+(1−P)+ . . . ]+T _(r) P(1−P)[1+2(1−P)+3(1−P)² . . . ]Substituting the sum of the geometric series:

${1 + x + x^{2} + x^{3} + \ldots} = \frac{1}{1 - x}$${{1 + {2x} + {3x^{2}} + \ldots} = \frac{1}{\left( {1 - x} \right)^{2}}},$One finds a simple expression for the average propagation delay on asingle point-to-point-link:

$D = {T_{d} + {T_{r}\left( \frac{1 - P}{P} \right)}}$

Due to the constantly changing environment, this average propagationdelay value should preferably be updated after each use of any givenlink. The expression of the average propagation delay as a function ofthe link utilization number n is written as follows:

${D(n)} = {T_{d} + {T_{r}\left( \frac{1 - {P(n)}}{P(n)} \right)}}$

This delay can be split in a static part and dynamic part:D(n)=T _(d) +T _(r) D _(r)(n)where D_(r)(n) is the part of the propagation delay due toretransmissions, normalized to the retransmission wait time. D_(r)(n) isdirectly related to the packet success rate by:

${D_{r}(n)} = \frac{1 - {P(n)}}{P(n)}$

After each packet transmission attempt on a given link, the packetsuccess rate P(n) is updated with a sliding average as shown below:

${{P(n)} = {\frac{{PS}(n)}{N_{av}} + {\frac{N_{av} - 1}{N_{av}}{P\left( {n - 1} \right)}}}},$where N_(av) is the number of transmissions used to compute the averageand PS(n) is the success/failure at attempt n:

${{PS}(n)} = \left\{ \begin{matrix}0 & {{if}\mspace{14mu}{transmission}\mspace{14mu} n\mspace{14mu}{failed}} \\1 & {{if}\mspace{14mu}{transmission}\mspace{14mu} n\mspace{14mu}{succeeded}}\end{matrix} \right.$

From the PSR update equation, one can derive an equation to update thepropagation delay:

${D_{r}(n)} = {{\frac{1}{P(n)} - 1} = {\frac{1}{\frac{{PS}(n)}{N_{av}} + {\frac{N_{av} - 1}{N_{av}}{P\left( {n - 1} \right)}}} - 1}}$Expressing the delay as a function of the PSR,

${P(n)} = \frac{1}{1 + {D_{r}(n)}}$we find

${D_{r}(n)} = {{\frac{1}{\frac{{PS}(n)}{N_{av}} + {\frac{N_{av} - 1}{N_{av}}\frac{1}{1 + {D_{r}\left( {n - 1} \right)}}}} - 1} = {\frac{N_{av}\left( {1 + {D_{\; r}\left( {n - 1} \right)}} \right)}{{{{PS}(n)}\left( {1 + {D_{r}\left( {n - 1} \right)}} \right)} + N_{av} - 1} - 1}}$

The equations to update the propagation delay of any link are then:

${D_{r}(n)} = \left\{ \begin{matrix}\frac{{N_{av}{D_{r}\left( {n - 1} \right)}} + 1}{N_{av} - 1} & {{{if}\mspace{14mu}{{PS}(n)}} = 0} \\\frac{\left( {N_{av} - 1} \right){D_{r}\left( {n - 1} \right)}}{{D_{r}\left( {n - 1} \right)} + N_{av}} & {{{if}\mspace{14mu}{{PS}(n)}} = 1}\end{matrix} \right.$

If a transmission attempt fails, we use the first equation; if itsucceeds, we use the second one. This can be easily extended to amulti-hop path in the following way:

${D(n)} = {{N_{hop}T_{d}} + {T_{r}{\sum\limits_{k = 1}^{N_{hop}}{D_{r,k}(n)}}}}$where the summation is extended to all the individual hops of a path.When this equation is used for the purpose of comparing different paths,it can be normalized in the following way:

$\frac{D(n)}{T_{r}} = {{N_{hop}\frac{T_{d}}{T_{r}}} + {\sum\limits_{k = 1}^{N_{hop}}{D_{r,k}(n)}}}$

Main benefits include: leads to an optimal path selection; and can beimplemented in each node with only the local knowledge of theneighborhood.

The following relates to present protocol subject matter regardingoperation for traffic load management, particularly including responsescenarios involving packet transmission failures.

In order to best convey present traffic management rules, one needs tomake several simplifying assumptions because the problem of RFcollisions is extremely complex. The issue at hand is not the same astrying to make accurate traffic load evaluations. Detailed traffic loadmanagement algorithms are set forth in the protocol descriptionotherwise herein.

To begin with, one may analyze the possible causes for a packettransmission failure and corresponding possible cure for each case,assuming in a first step that the cause of the failure is known. Thiswill consider analysis of the problem of traffic flow at the cell relayand derive a simple traffic limitation rule from this special case.Thereafter, a more global strategy can be outlined to deal with thesefailures through adequate wait times and retransmissions. This strategyis designed to optimize the latency and the throughput of individuallinks between nodes. It is not designed to evenly share the trafficbetween the different paths of the meshed network. The proposed loadmanagement will prevent the network from collapsing if demand exceedsthe network capacity, but it does not replace load management at theapplication level. Load management at the application layer is expectedto spread the traffic as evenly as possible in time and to avoidrequesting too much data at the same time. LAN network overload shouldremain an exceptional situation.

Present FIG. 74 is a table relating to various scenarios of transmissionfailure causes and solutions, per present protocol subject matter, whichare further commented on herein.

The following is with particular reference to the FIG. 74 “Possiblecause (1)” set of conditions. Rayleigh fading is strongly frequencydependent. As a consequence, some channels will have a much betterpacket success rate than others. This will be particularly noticeablewith long distance links with a weak link margin. It will not beuncommon to see more than 10-dB difference between the link budgets oftwo channels. Such a difference will make some channels unusable fordata transmission purpose, even when no jamming signals are present.Rayleigh fading is due to multi-path interference and the environmentalconditions that make a channel poor will vary in time and from node tonode. It is therefore not simple to exclude these bad channels from thelist. Furthermore, the radio regulations in the US prevent one fromdoing so entirely; all the channels must be used in the same way. When apacket fails to be acknowledged due to such conditions, the solution isto try again on another channel. There is no need to wait before thenext transmission attempt in this case. Additional wait time here wouldonly increase the system latency. In some extreme cases, the system willfail to transmit successfully its packets on all the available channels.This can be caused by harsh environmental conditions like a thunderstormor by the presence of a big obstruction close to the endpoint. Thiscondition can also come with a loss of synchronization. In this case,the endpoint will have to introduce a significant wait time and resumeits transmissions later, as outlined in the table of present FIG. 74.

The following is with particular reference to the FIG. 74 “Possiblecause. (2)” set of conditions. The ISM band used by the present protocolsubject matter is a shared band. Other users of the band will interferewith such transmissions and some packets will be lost due to collisions.Of course, this phenomenon will be more important for long distancelinks due to a weaker link margin. Interferers can be narrow-band lowpower transmitters, frequency hoppers or high-power wide-bandtransmitters. The bandwidth of any interferer will nonetheless be smallcompared to the whole ISM bandwidth and, in most cases, theretransmission on a different channel will be sufficient to avoid theinterference. As in the previous case there is no need to wait beforeattempting a retransmission. Additional wait time here would onlyincrease the system latency. In some extreme cases, the system will failto transmit successfully its packets on all the available channels. Thiscan be caused by a very high power jammer close enough to the endpointto de-sense its receiver front end. In this case, the endpoint will haveto introduce a significant wait time and resume its transmissions later,as outlined in the table of present FIG. 74.

The following is with particular reference to the FIG. 74 “Possiblecause (3)” set of conditions. As the traffic generated by the presentprotocol subject matter grows higher, internal collisions betweenpackets will occur. At some point, these internal collisions will befrequent enough to degrade the effective throughput of the system. Inthis case transmitting on another channel will not improve the situationbecause every endpoint follows the same hopping pattern. From acollision probability standpoint, the system behaves as if only onechannel was used. The relation between collision probability andeffective throughput is well known from slotted Aloha theory. Textbooktheory deals with the case where no external jammer is present. Here thesituation is more difficult to analyze because we have both types ofcollisions at the same type: internal collisions due to subject trafficand external collisions with the other users of the band. Hence, it isdesirable to introduce a regulation mechanism to slow down the subjecttraffic when it grows above some limit, as outlined in the table ofpresent FIG. 74.

The following is with particular reference to the FIG. 74 “Possiblecause (4)” set of conditions. When an endpoint cannot handle an incomingmessage due to memory limitations, it will reply with a negativeacknowledgment and discard the received message. This can also be causedby traffic congestion in a remote node of the network. When a node needsto slow down its traffic, it will reply with a negative acknowledgementwhen its input buffer fills up. This condition will propagate step bystep along the upstream traffic path. The recipient of a negativeacknowledgement should either try another destination or retransmitafter some wait time, as outlined in the table of present FIG. 74.

The following provides additional discussion with reference to analysisof the upload traffic flow at the cell relay, in accordance with presentprotocol subject matter. The cell relay is the point where all thetraffic converges. If a gridlock occurs, it will most likely occur atthe cell relay, it is therefore important to analyze the trafficconditions in this specific case. Only the upload situation isconsidered here because this is the relevant case for traffic loadmanagement. Present FIG. 75 diagrammatically represents a model for thetraffic load of the present subject matter, at the cell relay, anduseful for the reference in the present discussion.

As the actual situation is extremely complex, some simplifications aredesired in order to more readily characterize some present trafficmanagement rules. An assumption is made, for instance, that the cellrelay (endpoint A in present FIG. 75) can only hear the transmissionsfrom level 2 endpoints (endpoints B_(i) in FIG. 75). This is anidealized assumption. In an actual implementation, sporadic successfultransmissions between level 3 endpoints and the cell relay will morelikely occur. An assumption is also made that the level 2 endpoints areout of range of each other. This looks like an idealized situation forlevel 2 endpoints but it is a worst case from the cell relay standpointbecause a level 2 endpoint has no knowledge of the traffic sent by itsneighbors to the cell relay in this case. Another present simplifyingassumption is that two packets, two acknowledgements, or a packet and anacknowledgement arriving in a receiver in the same time slot will alwayscollide and result in nothing at all being received. This is of course apessimistic assumption, but only extensive simulations will allow a moreaccurate modeling of the collision process.

The following notation is utilized to describe the traffic flow:

-   -   R(X, Y, Z)=average number of packets sent by endpoint X,        intended for endpoint Y and above sensitivity threshold for        endpoint Z, by time slot    -   S(X, Y, Z)=average number of packets sent by endpoint X,        intended for endpoint Y and successfully received by endpoint Z,        by time slot    -   T(X, Y, Z)=average number of unique packets sent by endpoint X,        intended for endpoint Y and successfully received by endpoint Z,        by time slot (i.e. without duplicated packets)    -   U(X, Y, Z)=average number of acknowledgements sent by endpoint        X, intended for endpoint Y and above sensitivity threshold for        endpoint Z, by time slot    -   V(X, Y, Z)=average number of acknowledgements sent by endpoint        X, intended for endpoint Y and successfully received by endpoint        Z, by time slot

The total traffic input density seen by the cell relay is the sum of thetraffic densities generated by all the sons of the cell relay (endpointsB_(i) in FIG. 75). The total traffic breaks down in data packets andacknowledgements. The data traffic is given by:

${R\left( {B,A,A} \right)} = {\sum\limits_{i = 1}^{N}{R\left( {B_{i},A,A} \right)}}$where B means the set of all level 2 endpoints, B={B₁, B₂, . . . B_(N)}The sum extends over all the sons of A (the level 2 endpoints). Theacknowledgements sent by endpoint B_(i) and intended for the sons ofB_(i) are heard by A and must be included in the total traffic seen byA. This acknowledgement traffic is given by:

${U\left( {B,C,A} \right)} = {\sum\limits_{i = 1}^{N}{U\left( {B_{i},{{Sof}\left( B_{i} \right)},A} \right)}}$where C means the set of all level 3 endpoints, as given by

${{C\;\bullet}\underset{i = 1}{\bigcup\limits^{N}}{{Sof}\left( B_{i} \right)}},$with the notation Sof (X)@{all the sons of X}. For the purpose ofcollision probability evaluation, these two kinds of traffic will betreated in the same way. The total traffic is then R (B, A, A)+U (B, C,A).

The data packet throughput at the cell relay is given by:

${S\left( {B,A,A} \right)} = {\sum\limits_{i = 1}^{N}{{R\left( {B_{i},A,A} \right)}{{PSR}\left( {B_{i},A} \right)}}}$where we have introduced the packet success rate for the transmission ofa packet from endpoint B_(i) to endpoint A:

${{PSR}\left( {B_{i},A} \right)} = {{\exp\left\lbrack {{- {\sum\limits_{\overset{k = 1}{k \neq i}}^{N}\;{R\left( {B_{k},A,A} \right)}}} - {\sum\limits_{\overset{k = 1}{k \neq i}}^{N}\;{U\left( {B_{k},{{Sof}\left( B_{k} \right)},A} \right)}}} \right\rbrack}{{PSR}_{e}(A)}{Q(A)}}$

The first term in the expression of the PSR is the probability for allother level 2 endpoints to be silent when B_(i) transmits its packet.This probability is given by the Poisson distribution for “zero event”.The expression in brackets is the average value of the number of eventsin a single time slot. The next term, PSR_(e)(A), is the probability forthe data packet to be received without collision with interferersoutside the present subject matter network. Finally Q(A) is theprobability for endpoint A to be in the receiving state when the packetarrives. This probability is equal to one minus the probability forendpoint A to be acknowledging a previously received packet:Q(A)=1−S(B, A, A)

The cell relay will acknowledge each received packet, this provides arelation between the number of packets successfully received by A andthe number of acknowledgements arriving at B_(i):U(A, B _(i) , B _(i))=S(B _(i) , A, A)

The number of acknowledgements successfully received by B_(i) is givenby:

$\begin{matrix}{{V\left( {A,B_{i},B_{i}} \right)} = {{U\left( {A,B_{i},B_{i}} \right)}{{ASR}\left( {A,B_{i}} \right)}}} \\{= {{R\left( {B_{i},A,A} \right)}{{PSR}\left( {B_{i},A} \right)}{{ASR}\left( {A,B_{i}} \right)}}}\end{matrix}$where the acknowledgement success rate, ASR (A, B_(i)), is introduced.This acknowledgement success rate is given by:ASR(A, B _(i))=exp[−R(Sof(B _(i)), B _(i) , B _(i))−U(Sof(B _(i)), Sof²(B _(i)), B _(i))]PSR_(e)(B _(i))where it is defined that Sof²(B_(i))=Sof (Sof(B_(i))).

From the application standpoint the relevant traffic is the number ofpackets received by the cell relay after deletion of the duplicatedpackets. A duplicated packet is generated when the acknowledgement failsto be received by the sender of the packet. This number of uniquepackets should be equal to the number of acknowledgements successfullyreceived by the level 2 endpoints, T(B_(i), A, A)=V(A, B_(i), B_(i))

It follows a relation between T(B_(i), A, A) and R(B_(i), A, A):T(B _(i) , A, A)=R(B _(i) , A, A)PSR(B _(i) , A)ASR(A, B _(i))

In this equation PSR(B_(i), A)ASR(A, B_(i)) is the success rate for thetransmission of packet and the reception of the followingacknowledgement. This is the “packet success rate” that the endpointB_(i) will measure when trying to send its packet to the cell relay. Thetotal rate of reception of not duplicated packets is obtained by summingthe preceding equation:

${T\left( {B,A,A} \right)} = {\sum\limits_{i = 1}^{N}\;{{R\left( {B_{i},A,A} \right)}{{PSR}\left( {B_{i},A} \right)}{{ASR}\left( {A,B_{i}} \right)}}}$

The number of acknowledgements sent by A to B_(i) is related to thetraffic rate by:

${U\left( {A,B_{i},B_{i}} \right)} = {{{R\left( {B_{i},A,A} \right)}{{PSR}\left( {B_{i},A} \right)}} = \frac{T\left( {B_{i},A,A} \right)}{{ASR}\left( {A,B_{i}} \right)}}$

In a similar way, the acknowledgements received by the cell relay butintended for the level 3 endpoints are also proportional to the totaltraffic:

${U\left( {B_{i},{{Sof}\left( B_{i} \right)},A} \right)} = \frac{T\left( {{{Sof}\left( B_{i} \right)},B_{i},B_{i}} \right)}{{ASR}\left( {B_{i},{{Sof}\left( B_{i} \right)}} \right)}$

The total contribution of acknowledgements to the traffic at the cellrelay is then:

${U\left( {B,C,A} \right)} = {\sum\limits_{i = 1}^{N}\;\frac{T\left( {{{Sof}\left( B_{i} \right)},B_{i},B_{i}} \right)}{{ASR}\left( {B_{i},{{Sof}\left( B_{i} \right)}} \right)}}$

To make the problem tractable, it is necessary to make furtherassumptions. A further such assumption is that the number of sons of Ais large. In this case, the individual contributions of each B_(i) tothe total traffic is small and the packet success rate becomesindependent of i as follows:PSR(B, A)=exp[−R(B, A, A)−U(B, C, A)]PSR_(e)(A)Q(A)

To simplify the expression for the acknowledgement success rate, we willnotice that implementation of the present protocol subject matter doesnot result in a pure Aloha system. When a son of a level 2 endpointhears its father sending a packet, it will postpone its own transmissionto avoid interfering with the acknowledgement its father is expecting.The probability for B_(i) to receive in the same time slot anacknowledgement from one its sons (intended for its grandson) and anacknowledgement from A is also much smaller than previously assumed.This would only occur if, in the previous time slot, B_(i) had sent apacket to A and the son of B_(i) had also sent a packet to its own son.This is likely to produce a collision. The acknowledgement success ratewill therefore be approximated with:ASR(A, B)=PSR_(e)where we further assumed that the external collision rate is the samefor all endpoints.

The relation between the throughput and the input traffic density at thecell relay simplifies to:T(B, A, A)=PSR(B, A)ASR(A ,B)R(B, A, A)

The acknowledgment rate from level 2 to level 3 is approximated in thesame way:ASR(B _(i) , Sof(B _(i)))=PSR_(e)

And the acknowledgement input density at the cell relay becomes:

${U\left( {B,C,A} \right)} = {\frac{\sum\limits_{i = 1}^{N}\;{T\left( {{{Sof}\left( B_{i} \right)},B_{i},B_{i}} \right)}}{{PSR}_{e}} = \frac{T\left( {B,A,A} \right)}{{PSR}_{e}}}$where one uses the conservation of the number of packets from level tolevel. The PSR from B to A can now be expressed as:

${{PSR}\left( {B,A} \right)} = {{\exp\left\lbrack {{- {R\left( {B,A,A} \right)}} - \frac{T\left( {B,A,A} \right)}{{PSR}_{e}}} \right\rbrack}{{PSR}_{e}\left\lbrack {1 - \frac{T\left( {B,A,A} \right)}{{PSR}_{e}}} \right\rbrack}}$

After substitution in the relation between T(B, A, A) and R(B, A, A),the following is obtained:

${T\left( {B,A,A} \right)} = {{R\left( {B,A,A} \right)}{\exp\left\lbrack {{- {R\left( {B,A,A} \right)}} - \frac{T\left( {B,A,A} \right)}{{PSR}_{e}}} \right\rbrack}{{PSR}_{e}^{2}\left\lbrack {1 - \frac{T\left( {B,A,A} \right)}{{PSR}_{e}}} \right\rbrack}}$

This can be written in this way:

$\frac{\frac{T\left( {B,A,A} \right)}{{PSR}_{e}}{\exp\left\lbrack \frac{T\left( {B,A,A} \right)}{{PSR}_{e}} \right\rbrack}}{{PSR}_{e}\left\lbrack {1 - \frac{T\left( {B,A,A} \right)}{{PSR}_{e}}} \right\rbrack} = {{R\left( {B,A,A} \right)}{\exp\left\lbrack {- {R\left( {B,A,A} \right)}} \right\rbrack}}$

The left-hand side of this equation is a monotonic function of T(B, A,A), the right-hand side has a maximum value for R(B, A, A)=1, it followsan equation that can be solved to find the maximum possible value ofT(B, A, A):

${{\frac{T\left( {B,A,A} \right)}{{PSR}_{e}}{\exp\left\lbrack {1 + \frac{T\left( {B,A,A} \right)}{{PSR}_{e}}} \right\rbrack}} - {{PSR}_{e}\left\lbrack {1 - \frac{T\left( {B,A,A} \right)}{{PSR}_{e}}} \right\rbrack}} = 0$

Present FIGS. 76 and 77, respectively, illustrate various aspects ofsuch relations. In particular, present FIG. 76 graphically illustratesData throughput, T(B, A, A) and PSR (with acknowledgement) vs trafficinput density, R(B, A, A) for PSRe=0.8, while present FIG. 75graphically illustrates Data throughput, T(B, A, A) vs PSRe. In anyevent, whatever the external collision rate, the maximum throughput isalways obtained for R(B, A, A)=1. Such interesting feature is used fortraffic load management per the present protocol subject matter.

In contrast to the foregoing discussion concerning analysis of theupload traffic flow at the cell relay, in accordance with the presentprotocol subject matter, the following more particularly relates toevaluation of such upload traffic flow.

In order to control properly the traffic load per present subjectmatter, an endpoint needs to evaluate the amount of traffic goingthrough the network. From electromagnetic theory, it is known that anytransmission path from antenna to antenna is reciprocal. For simplicity,one may make the additional assumption that the links are balanced, thatis, the transmitted powers and sensitivities are the same for allendpoints. One can then say that if node A is a father for node B_(i),every packet transmitted by A will be heard by B_(i), with theexceptions of the packets lost by collision or because B_(i) was not inreception mode at the right moment. This provides node B_(i) with asimple, approximate way to know the amount of traffic its father ishandling at any moment. Node B_(i) needs to listen to theacknowledgments transmitted by the cell relay. The acknowledgments willgive enough information to assess the traffic load.

For each received packet, the cell relay sends an acknowledgement. It istherefore calculated that U(A, B, B_(i))=S(B, A, A). The rate ofacknowledgements successfully received by B_(i) is given by:V(A, B, B _(i))=U(A, B, B _(i))ASR(A, B _(i))Q(B _(i))where Q(B_(i)) is included because endpoint B_(i) is monitoringacknowledgements intended for other endpoints and is not always inlistening state when these acknowledgements arrive.

It is known that the traffic input density is given by

R(B, A, A)=S(B, A, A)/PSR(A, B). The relation between R(B, A, A) andV(A, B, B_(i)) is:

${R\left( {B,A,A} \right)} = \frac{V\left( {A,B,B_{i}} \right)}{{{PSR}\left( {A,B} \right)}{{ASR}\left( {A,B_{i}} \right)}{Q\left( B_{i} \right)}}$

When measuring V (A, B, B_(i)), one should consider all acknowledgements(positive or negative) sent by A and received by B_(i). But theacknowledgements intended for B_(i) are not taken into account in thiscomputation. Only the acknowledgements addressed to other endpoints arerecorded here because the purpose is to evaluate the external subjecttraffic the endpoint A has to deal with. V (A, B, B_(i)) can be measuredby counting all the received acknowledgements occurring in a slidingtime window. If one expresses the packet success rate in terms of theaverage propagation delay, the result is as follows:

${{{PSR}\left( {B,A} \right)}{{ASR}\left( {A,B} \right)}} = \frac{1}{1 + {LPD}_{BA}}$

The traffic input density can be seen by endpoint A as a function of thepropagation delay and the rate of reception of acknowledgments. Thefollowing equation will indicate how busy is endpoint A:

${R\left( {B,A,A} \right)} = \frac{{V\left( {A,B,B_{i}} \right)}\left( {1 + {LPD}_{BA}} \right)}{Q\left( B_{i} \right)}$

The following more particularly relates to traffic management algorithmsper the present protocol subject matter. When the LLC layer receivesfrom the NET layer a request to send a packet, or when it reschedules anon-acknowledged transmission, it will compute the length of the waittime (Tx_Wait) before the request to send can be forwarded to the MAClayer. This wait time is computed as a function of the repetitionnumber. The purpose of such approach is to avoid flooding the airinterface with a large number of packets when the conditions oftransmission are difficult.

When the MAC layer receives a request to send a packet from the LLClayer, the randomization window length (Tx_Window) is computed as afunction the traffic load, its purpose is to avoid using the slottedAloha interface above its optimal point. The transmission of a packetwill always occur within the randomization window, after the wait time.Such facet of the present subject matter is illustrated by present FIG.80, which graphically represents the wait time and randomization windowfor the (re-)transmission of a packet. Such wait time is preferablycomputed according to a binary exponential backoff law, as otherwiseexplained herein.

The following more particularly addresses advantageous mitigation ofcollision circumstances, per present subject matter. In this context,the simultaneous (or overlapping) reception of two or more packets isreferred to as a collision. If the colliding packets have the samepower, both will be lost. If one packet is received with a higher power(higher than some carrier to interference ratio) and if the morepowerful packet arrives first, the stronger packet is receivedsuccessfully and the weaker one is lost. Such a set of conditions and/orevents are represented by present FIG. 79, that is, representing such acollision episode where one packet (designated as packet 1) is lost.

If the weaker packet arrives first, two scenarios are possible,depending on the type of receiver used in the pertinent portion of theimplementation. If the receiver has relatively more limitedfunctionalities, it will lock on the preamble of the first packet, gointo sync word search and then into a demodulation phase. When thestronger packet arrives, the receiver is not in a preamble search stateand misses the stronger packet sync word. Both packets are lost. Such aset of conditions and/or events are represented by present FIG. 80, thatis, representing a collision episode where both packets (designated aspacket 1 and packet 2) are lost. In the situation where the receivedhappens to be a more sophisticated device, the receiver is able todemodulate a packet and concurrently search for a new preamble. In suchcase, it can receive the stronger packet. The weaker one (packet 1) ishowever lost in all cases.

To mitigate the probability of collision with any kind of receiver, itcan be useful per present subject matter to avoid using the first subtime slot for acknowledgments. Such preferred approach will avoid thedestruction of some packets by weaker acknowledgements arriving justbefore the packet.

While the present subject matter has been described in detail withrespect to specific embodiments thereof, it will be appreciated thatthose skilled in the art, upon attaining an understanding of theforegoing may readily produce alterations to, variations of, andequivalents to such embodiments. Accordingly, the scope of the presentdisclosure is by way of example rather than by way of limitation, andthe subject disclosure does not preclude inclusion of suchmodifications, variations and/or additions to the present subject matteras would be readily apparent to one of ordinary skill in the art.

In addition, various discussion herein makes us of and/or relies onabbreviations and acronyms, having the intended meanings as set forth inthe following Table of Definitions, which forms part of the presentdisclosure.

Table of Definitions ACK Acknowledgement AMI Advanced MeteringInitiative API Application Layer Interface C12.22 ANSI standard protocolfor interfacing to data communication networks. It is the recommendedAPI protocol to be used with Linknet protocol. CM Cell Master CR CellRelay CRC Cyclic Redundancy Check ERC European RadiocommunicationsCommittee EP Endpoint, network node EP_GPD Global average PropagationDelay between an endpoint and the Cell Master through a specifiedneighbor. ETSI European Telecommunications Standards Institute FCCFederal Communications Commission FCS Frame Check Sequence FEC ForwardError Correction FH Frequency Hopping FHSS Frequency Hopping SpreadSpectrum GF Galois Field GPD Global average Propagation Delay between anendpoint and the Cell Master (the minimum EP_GPD for an endpoint) IEEEInstitute of Electrical and Electronics Engineers ISM Industrial,Scientific and Medical ISO International Standards Organization ITPItron Time Protocol Linknet The name of the protocol described in thepresent document LLC Logical Link Control layer LPDU LLC PDU LPD LocalPD. Propagation Delay between two neighboring EPs MAC Medium AccessControl layer MPDU MAC PDU NACK Negative acknowledgement NET Networklayer NETPDU NET PDU OSI Open Systems Interconnection PDF ProbabilityDensity Function PDU Protocol Data Unit PHY Physical layer PPDU Physicallayer PDU PSR Packet Success Rate RITP Relative Itron Time Protocol RSReed-Solomon or Registered State RSSI Received Signal Strength IndicatorRTOS Real Time Operating System SAP Service Access Point SFD Start ofFrame Delimiter TS Time Slot RF2Net Previous project name for Linknetdevelopment WAN Wide Area Network Zigbee Standard IEEE protocol.

1. A method for an advanced metering system mesh network with multiplemode downlink routing, comprising: establishing a network including acentral facility and a plurality of node devices, with at least some ofthe node devices comprising metrology devices, and with at least some ofthe node devices comprising cell masters which provide communicationsbetween the central facility and others of the node devices associatedtherewith in respective cells formed by each respective cell master;configuring the network for bi-directional communications between thecentral facility and each of the plurality of node devices; from eachnode device associated with a given specific cell master in a respectivecell formed thereby, providing information to the given specific cellmaster about communications links of each of the node devices in therespective cell; selectively sending a monocast downlink message from agiven respective cell master to a specific node device associatedtherewith, using a monocast downlink path determined by the givenrespective cell master based on its received information from all nodedevices associated therewith; and selectively sending a broadcastdownlink message from a given respective cell master to all node devicesassociated therewith; and wherein each node device provides updatedinformation to its respective cell master about any changes to itscommunications links in the respective cell; each node device returns aBroken Link message to its respective cell master if it is unable aftera predetermined number of retransmissions to successfully transmit amessage to its intended recipient; and each cell master uses Broken Linkmessages and updated information received from the respective nodedevices in its respective cell in its subsequent determinations of anydownlink monocast paths.
 2. A method as in claim 1, wherein each of thenode devices: retransmits incoming broadcast downlink messages butfilters incoming broadcast downlink messages so as to delete anybroadcast downlink message which it has already retransmitted so as toavoid duplicative retransmissions; retransmits up to a predeterminednumber of times any message transmitted thereby which fails to beacknowledged by its intended recipient, until the message issuccessfully transmitted; and filters from any retransmissions anymessage which is beyond a predetermined period of time after itsoriginal transmission from a cell master associated with the nodedevice.
 3. A method for an advanced metering system frequency hoppingmesh network with dynamic multiple mode downlink routing to all nodedevices thereof, comprising: establishing a network including a centralfacility and a plurality of node devices, with at least some of the nodedevices comprising metrology devices, with at least some of the nodedevices comprising cell masters which provide communications between thecentral facility and others of the node devices associated therewith inrespective cells formed by each respective cell master, and with atleast some of the node devices comprising father node devices whichprovide a communications link between a respective cell master andothers of the node devices defining son node devices of a givenrespective father node device in the respective cell of the respectivecell master; configuring the network for bi-directional communicationsbetween the central facility and each of the plurality of node devicesvia associations with respective of the cell masters; at each nodedevice, maintaining a neighbor table containing information about eachnode device and its respective neighbor relationships and communicationslinks within a respective cell formed by a respective cell master; fromeach node device associated with a given specific cell master in arespective cell formed thereby, forwarding the neighbor table thereof tothe given specific cell master; at each node device associated with agiven specific cell master in a respective cell formed thereby, updatingits respective neighbor table when any changes occur to any of itsrespective neighbor relationships or communications links, andforwarding the updated neighbor table thereof to the given specific cellmaster; selectively sending a broadcast downlink message from a givenrespective cell master to all node devices associated therewith in itsrespective cell; and selectively sending a monocast downlink messagefrom a given respective cell master to a specific node device associatedtherewith in the respective cell formed by the given respective cellmaster, using a monocast downlink path determined by the givenrespective cell master based on received neighbor tables from all nodedevices associated therewith in its respective cell, whereby the networkdynamically adapts its downlink routing to changes in communicationslinks among the node devices thereof.
 4. A method as in claim 3, furthercomprising at each cell master, building a routing table thereat fordetermining a monocast downlink path to each respective node devicewithin its respective cell, based on a predetermined minimum number ofneighbor tables from the respective node devices within its respectivecell, and updating the routing table based on any received informationregarding changes to any of the respective node devices or therespective neighbor relationships or communications links thereof.
 5. Amethod as in claim 4, wherein the monocast downlink paths are determinedso as to avoid for a given path multiple passes through the same nodedevice, so as to avoid a circular path.
 6. A method as in claim 3,wherein: the monocast downlink paths respectively comprise a list ofeach interim node device along a given path within a respective cell,for reaching a particular node device within the cell; and each interimnode device along a given monocast downlink path removes its identityfrom the path list and retransmits the monocast downlink message to anext identified node device on the path list.
 7. A method as in claim 6,wherein: each node device along a given monocast downlink path sends aBroken Link return message to the cell master associated with it in itsrespective cell, if the node device is unable to successfully retransmitsaid monocast downlink message to said next identified node device onthe path list; and upon receipt of a Broken Link message at a respectivecell master, the cell master redetermines a monocast downlink path, ifpossible based on available information about its respective cell, to aparticular node device, and retransmits the respective monocast downlinkmessage along the redetermined monocast downlink path.
 8. A method as inclaim 7, wherein a node device which is unable to successfully transmita respective monocast downlink message to the next identified nodedevice on the path list does not send a Broken Link return message tothe cell master associated with it in its respective cell if therespective monocast downlink message is either of a notification that aparticular node device within the respective cell is out or anotification that a particular node device is newly registered to therespective cell.
 9. A method as in claim 3, wherein each of the nodedevices: retransmits incoming broadcast downlink messages but filtersincoming broadcast downlink messages so as to delete any broadcastdownlink message which it has already retransmitted so as to avoidduplicative retransmissions; retransmits up to a predetermined number oftimes any message transmitted thereby which fails to be acknowledged byits intended recipient, until the message is successfully transmitted;and filters from any retransmissions any message which is beyond apredetermined period of time after its original transmission from a cellmaster associated with the node device.
 10. An advanced metering systemmesh network with multiple mode downlink routing, comprising: a centralfacility; and a plurality of node devices, with at least some of thenode devices comprising metrology devices, and with at least some of thenode devices comprising cell masters which provide communicationsbetween said central facility and others of said node devices associatedtherewith in respective cells formed by each respective cell master,with each node device configured for bi-directional communications withsaid central facility; wherein each of said node devices is configuredto provide information to its respective cell master aboutcommunications links thereof in the respective cell; and each respectivecell master is configured to selectively send a monocast downlinkmessage from it to a specific node device associated therewith, using amonocast downlink path determined by it based on its receivedinformation from all node devices associated therewith, and is furtherconfigured to selectively send a broadcast downlink message from it toall node devices associated therewith; wherein each of said node devicesis further configured for providing updated information to itsrespective cell master about any changes to its communications links inthe respective cell, and for returning a Broken Link message to itsrespective cell master if it is unable after a predetermined number ofretransmissions to successfully transmit a message to its intendedrecipient; and each respective cell master is further configured forusing Broken Link messages and updated information received from therespective node devices in its respective cell in its subsequentdeterminations of any downlink monocast paths.
 11. An advanced meteringsystem mesh network as in claim 10, wherein each of said node devices isfurther configured for: retransmitting incoming broadcast downlinkmessages but filtering incoming broadcast downlink messages so as todelete any broadcast downlink message which it has already retransmittedso as to avoid duplicative retransmissions; retransmitting up to apredetermined number of times any message transmitted thereby whichfails to be acknowledged by its intended recipient, until the message issuccessfully transmitted; and filtering from any retransmissions anymessage which is beyond a predetermined period of time after itsoriginal transmission from a cell master associated with the nodedevice.
 12. An advanced metering system frequency hopping mesh networkwith dynamic multiple mode downlink routing to all node devices thereof,comprising: a central facility; and a plurality of node devices, with atleast some of the node devices comprising metrology devices, with atleast some of the node devices comprising cell masters which providecommunications between said central facility and others of said nodedevices associated therewith in respective cells formed by eachrespective cell master, and with at least some of the node devicescomprising father node devices which provide a communications linkbetween a respective cell master and others of said node devicesdefining son node devices of a given respective father node device inthe respective cell of the respective cell master, with each of saidnode devices configured for bi-directional communications between saidcentral facility and each of said plurality of node devices viaassociations with respective of said cell masters; wherein each of saidnode devices is configured for maintaining a neighbor table containinginformation about its respective neighbor relationships andcommunications links within a respective cell formed by its respectivecell master, for forwarding its neighbor table to its respective cellmaster, for updating its respective neighbor table when any changesoccur to any of its respective neighbor relationships or communicationslinks, and for forwarding the updated neighbor table to its respectivecell master; and each respective cell master is configured forselectively sending a broadcast downlink message to all node devices inits respective cell, and for selectively sending a monocast downlinkmessage to a specific node device associated in its respective cell,using a monocast downlink path determined by it based on receivedneighbor tables from all node devices in its respective cell, wherebysaid network dynamically adapts its downlink routing to changes incommunications links among said node devices thereof.
 13. An advancedmetering system frequency hopping mesh network as in claim 12, whereineach respective cell master is further configured for building a routingtable thereat for determining a monocast downlink path to eachrespective node device within its respective cell, based on apredetermined minimum number of neighbor tables from the respective nodedevices within its respective cell, and updating the routing table basedon any received information regarding changes to any of the respectivenode devices or the respective neighbor relationships or communicationslinks thereof.
 14. An advanced metering system frequency hopping meshnetwork as in claim 13, wherein each respective cell master is furtherconfigured for determining monocast downlink paths so as to avoid for agiven path multiple passes through the same node device, so as to avoida circular path.
 15. An advanced metering system frequency hopping meshnetwork as in claim 12, wherein: each respective cell master is furtherconfigured for comprising the monocast downlink paths respectively as alist of each interim node device along a given path within a respectivecell, for reaching a particular node device within the cell; and eachrespective interim node device along a given monocast downlink path isfurther configured removing its identity from the path list andretransmitting the monocast downlink message to a next identified nodedevice on the path list.
 16. An advanced metering system frequencyhopping mesh network as in claim 15, wherein: each node device along agiven monocast downlink path is further configured for sending a BrokenLink return message to its cell master, if the node device is unable tosuccessfully retransmit said monocast downlink message to said nextidentified node device on the path list; each respective cell master,upon receipt of a Broken Link message, is further configured forredetermining a monocast downlink path, if possible based on availableinformation about its respective cell, to a particular node device, andfor retransmitting the respective monocast downlink message along theredetermined monocast downlink path.
 17. An advanced metering systemfrequency hopping mesh network as in claim 16, wherein a node devicewhich is unable to successfully transmit a respective monocast downlinkmessage to the next identified node device on the path list is furtherconfigured for not sending a Broken Link return message to its cellmaster if the respective monocast downlink message is either of anotification that a particular node device within the respective cell isout or a notification that a particular node device is newly registeredto the respective cell.
 18. An advanced metering system frequencyhopping mesh network as in claim 12, wherein each of said node devicesare further configured for: retransmitting incoming broadcast downlinkmessages but filtering incoming broadcast downlink messages so as todelete any the broadcast downlink message which it has alreadyretransmitted so as to avoid duplicative retransmissions; retransmittingup to a predetermined number of times any message transmitted therebywhich fails to be acknowledged by its intended recipient, until themessage is successfully transmitted; and filtering from anyretransmissions any message which is beyond a predetermined period oftime after its original transmission from its respective cell master.19. A metrology device for use with an advanced metering system meshnetwork with multiple mode downlink routing, and including a centralfacility, a plurality of node devices, with at least some of the nodedevices comprising metrology devices, and at least some of the nodedevices comprising cell masters providing communications between thecentral facility and others of the node devices associated therewith inrespective cells formed by the respective cell masters, with each nodedevice configured for bi-directional communications with the centralfacility via associations with respective cell masters, said metrologydevice comprising: a metrology portion configured to measure consumptionof a utility commodity; a transmitter portion configured to transmitconsumption information and other data; and a receiver portionconfigured to receive information from other network devices in anassociated network; wherein said metrology portion is configured formaintaining a neighbor table containing information about its respectiveneighbor relationships and communications links within a respective cellformed by its respective cell master in an associated network, forforwarding its neighbor table to its respective cell master, forupdating its respective neighbor table when any changes occur to any ofits respective neighbor relationships or communications links, and forforwarding the updated neighbor table to its respective cell master, sothat the cell master selectively forwards downlink messages to therespective node devices of its cell for arriving at an intended nodedevice.
 20. A metrology device as in claim 19, wherein said metrologyportion is further configured for receiving monocast downlink messagesfrom its respective cell master in an associated network, the monocastdownlink messages being passed along a monocast downlink path on whichsaid metrology device resides, and for removing its identity from alisting of the path included with its corresponding message andretransmitting the monocast downlink message to a next identified nodedevice on the listing of the path.
 21. A metrology device as in claim19, wherein said metrology portion is further configured for sending aBroken Link return message to its cell master, if the metrology deviceis unable to successfully transmit a downlink message to a nextidentified node device on a path list associated with the message.
 22. Ametrology device as in claim 21, wherein said metrology portion isfurther configured for not sending a Broken Link return message to itscell master in the event of an unsuccessful transmission of a downlinkmessage if the downlink message is either of a notification that aparticular node device within the respective cell is out or anotification that a particular node device is newly registered to therespective cell.
 23. A metrology device as in claim 19, wherein saidmetrology portion is further configured for: retransmitting incomingbroadcast downlink messages coming from its respective cell master in anassociated network but filtering incoming broadcast downlink messages soas to delete any broadcast downlink message which it has alreadyretransmitted so as to avoid duplicative retransmissions; retransmittingup to a predetermined number of times any message transmitted therebywhich fails to be acknowledged by its intended recipient, until themessage is successfully transmitted; and filtering from anyretransmissions any message which is beyond a predetermined period oftime after its original transmission from its respective cell master.